Category Archives: SQL

[mysql] BLOB 에 데이터 효율적으로 쑤셔넣기

일단 BLOB 사이즈 부터

TINYBLOB = 2^8(255B)

BLOB = 2^16(64K)

MEDIUMBLOB = 2^24(16M)

LONGBLOB = 2^32 (4G)


바이너리를 디비에 저장해야할 일이 생겼는데

사용하는 언어의 라이브러리가 별도로 BLOB형식을 지원해주지 않는경우에는

대부분 unpack 을 사용하여 hex 스트링으로 저장하여 저장공간을 두배나 차지하게되고

또한 사용시에도 pack 으로 다시 바이너리로 변환하여야하여 번거로운 방법을 사용하더라…


그럴필요없이 난


sql = "INSERT INTO apks (siteno, date, filename,filesize, orignname, package_name, package_version, md5, sha2 ,rsa) VALUES ('#{@siteno}', now(),'#{File.basename(filename)}', '#{filesize}' ,'#{File.basename(file)}', '#{packagename}', '#{packageversion}', '#{md5}', '#{sha2}' ,0x%s)" % rsa.unpack('H*')



이런식으로 저장하고 DB에 이쁘게 들어간것을 확인하였다.

Mysql Alter table

Alter Table은?

테이블에 새로운 컬럼을 추가하거나 삭제하는등의 작업을 하기 위해 필요한 MySQL의 Alter Table 문법입니다.


   alter_specification [, alter_specification] …

   table_option … | ADD [COLUMN] col_name column_definition
       [FIRST | AFTER col_name ]
  | ADD [COLUMN] (col_name column_definition,…)
  | ADD {INDEX|KEY} [index_name]
       [index_type] (index_col_name,…) [index_type]
       [index_type] (index_col_name,…) [index_type]
  | ADD [CONSTRAINT [symbol]]
       UNIQUE [INDEX|KEY] [index_name]
       [index_type] (index_col_name,…) [index_type]
       (index_col_name,…) [index_type]
  | ADD [CONSTRAINT [symbol]]
       FOREIGN KEY [index_name] (index_col_name,…)
  | ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  | CHANGE [COLUMN] old_col_name new_col_name column_definition
       [FIRST|AFTER col_name]
  | MODIFY [COLUMN] col_name column_definition
       [FIRST | AFTER col_name]
  | DROP [COLUMN] col_name | DROP PRIMARY KEY | DROP {INDEX|KEY} index_name
  | DROP FOREIGN KEY fk_symbol
  | RENAME [TO] new_tbl_name
  | ORDER BY col_name [, col_name] …
  | CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
  | [DEFAULT] CHARACTER SET [=] charset_name [COLLATE [=] collation_name]

   col_name [(length)] [ASC | DESC]


사용 예(Example)

1. 테이블에 새로운 컬럼 추가

alter table tablename add column [추가할 컬럼명] [추가할 컬럼 데이타형]

2. 테이블에 컬럼타입 변경하기

alter table tablename modify column [변경할 컬럼명] [변경할 컬럼 타입]

3. 테이블에 컬럼이름 변경하기

alter table tablename change column [기존 컬럼명] [변경할 컬럼명] [변경할 컬럼타입]

4. 테이블에 컬럼 삭제하기

alter table tablename drop column [삭제할 컬럼명]

5. 테이블컬럼에 인덱스 주기

alter table tablename add index 인덱스명(인덱스를 줄 컬럼1 , 인덱스를 줄 컬럼2, … )

6. 테이블컬럼에 인덱스 삭제하기

alter table tablename drop index 인덱스명;

7. 테이블에 Primary Key 만들기

alter table tablename add primary key (키를 줄 컬럼명1 , 키를 줄 컬럼명2, …)

8. 테이블에 Primary Key 삭제하기

alter table tablename drop primary key;

9. 테이블명 바꾸기

alter table 기존테이블명 rename 새로운테이블명

10. 인덱스 생성

CREATE [UNIQUE] INDEX index_name ON tbl_name (col_name[(length]),… )

11. 인덱스 삭제

DROP INDEX index_name


12.2.11. UPDATE Syntax

Single-table syntax:

Multiple-table syntax:

For the single-table syntax, the
UPDATE statement updates columns of
existing rows in tbl_name with new values. The
SET clause indicates which columns to modify
and the values they should be given. Each value can be given as an
expression, or the keyword DEFAULT to set a
column explicitly to its default value. The
WHERE clause, if given, specifies the
conditions that identify which rows to update. With no
WHERE clause, all rows are updated. If the
ORDER BY clause is specified, the rows are
updated in the order that is specified. The
LIMIT clause places a limit on the number of
rows that can be updated.

For the multiple-table syntax,
UPDATE updates rows in each table
named in table_references that satisfy
the conditions. In this case, ORDER BY and
LIMIT cannot be used.

where_condition is an expression that
evaluates to true for each row to be updated. It is specified as
described in Section 12.2.8, “SELECT Syntax”.

The UPDATE statement supports the
following modifiers:

  • If you use the LOW_PRIORITY keyword,
    execution of the UPDATE is
    delayed until no other clients are reading from the table.
    This affects only storage engines that use only table-level
    locking (MyISAM, MEMORY,

  • If you use the IGNORE keyword, the update
    statement does not abort even if errors occur during the
    update. Rows for which duplicate-key conflicts occur are not
    updated. Rows for which columns are updated to values that
    would cause data conversion errors are updated to the closest
    valid values instead.

If you access a column from tbl_name in
an expression, UPDATE uses the
current value of the column. For example, the following statement
sets the age column to one more than its
current value:

Single-table UPDATE assignments are
generally evaluated from left to right. For multiple-table
updates, there is no guarantee that assignments are carried out in
any particular order.

If you set a column to the value it currently has, MySQL notices
this and does not update it.

If you update a column that has been declared NOT
by setting to NULL, the column
is set to the default value appropriate for the data type and the
warning count is incremented. The default value is
0 for numeric types, the empty string
('') for string types, and the
zero” value for date and time types.

UPDATE returns the number of rows
that were actually changed. The
mysql_info() C API function
returns the number of rows that were matched and updated and the
number of warnings that occurred during the

You can use LIMIT
to restrict the
scope of the UPDATE. A
LIMIT clause is a rows-matched restriction. The
statement stops as soon as it has found
row_count rows that satisfy the
WHERE clause, whether or not they actually were

If an UPDATE statement includes an
ORDER BY clause, the rows are updated in the
order specified by the clause. This can be useful in certain
situations that might otherwise result in an error. Suppose that a
table t contains a column id
that has a unique index. The following statement could fail with a
duplicate-key error, depending on the order in which rows are

For example, if the table contains 1 and 2 in the
id column and 1 is updated to 2 before 2 is
updated to 3, an error occurs. To avoid this problem, add an
ORDER BY clause to cause the rows with larger
id values to be updated before those with
smaller values:

You can also perform UPDATE
operations covering multiple tables. However, you cannot use
ORDER BY or LIMIT with a
multiple-table UPDATE. The
table_references clause lists the
tables involved in the join. Its syntax is described in
Section, “JOIN Syntax”. Here is an example:

The preceding example shows an inner join that uses the comma
operator, but multiple-table UPDATE
statements can use any type of join allowed in
SELECT statements, such as

You need the UPDATE privilege only
for columns referenced in a multiple-table
UPDATE that are actually updated.
You need only the SELECT privilege
for any columns that are read but not modified.

If you use a multiple-table UPDATE
statement involving InnoDB tables for which
there are foreign key constraints, the MySQL optimizer might
process tables in an order that differs from that of their
parent/child relationship. In this case, the statement fails and
rolls back. Instead, update a single table and rely on the
ON UPDATE capabilities that
InnoDB provides to cause the other tables to be
modified accordingly. See
Section, “FOREIGN KEY Constraints”.

Currently, you cannot update a table and select from the same
table in a subquery.

User Comments

Posted by Vjero Fiala on December 6 2003 1:21am [Delete] [Edit]

Update one field with more fields from another table

Table A

Table B:

I will update field text in table A
UPDATE Table A,Table B
SET Table A.text=concat_ws(",Table A.text,Table B.B-num," from ",Table,'/')
WHERE Table A.A-num = Table B.A-num

and come to this result
Table A

(only one field from Table B is accepted)

But i will come to this result
Table A

Posted by Babu Ramesh on January 12 2004 11:33pm [Delete] [Edit]

Update column in a table whose values are not found in another table.


An outerjoin is performed based on the equijoin condition.
Records not matching the equijoin from table2 are marked with null.

This facilitates to update table1 column with expression whose corresponding value from table2 is returned as NULL

Posted by Mike Zhang on March 3 2004 5:32pm [Delete] [Edit]

update tab1, tab2 set, where

update tab2, tab1 set, where

former leads to the values of column "name" in two tables are equal and
equal to the original The latter, however, swaps the values
of column "name" in table tab1 with tab2.

If update (in two or
more tables in ONE sql) many columns simultaneously by means of more
complicated algorithm, eg., the value of one column is determined by
other serval columns, things are worse. I can not find the clear rule
described as above, and have to write the sql in two sentances
eventurally. ABSOLUTELY it is a bug.

As a preparation to sql
store procedure, multi-table update can drastically accelerate the bulk
update operation. Such an important feature added in version 4.0 is so
dangerous that you should verify the correctness of every mutili-table
update sql. MySQL develop group dare declare 4.0.15 is the standard
version! :(

Posted by Adam Boyle on March 2 2004 2:28pm [Delete] [Edit]

took me a few minutes to figure this out, but the syntax for UPDATING
ONE TABLE ONLY using a relationship between two tables in MySQL 4.0 is
actually quite simple:

update t1, t2 set t1.field = t2.value where t1.this = t2.that;

Posted by Neil Yalowitz on March 30 2004 7:56am [Delete] [Edit]

should be noted that even simple applications of UPDATE can conflict
with the 'safe mode' setting of the mysql daemon. Many server admins
default the MySQL daemon to 'safe mode'.

If UPDATE gives an error like this:

"You are using safe update mode and you tried to update a table without…etc."

it may be that your .cnf file must be edited to disable safemode. This
worked for me. In order for the change in the .cnf file to take effect,
you must have permission to restart mysqld in the server OS
environment. There is a page in the online documentation that explains
safe mode entitled 'safe Server Startup Script'.

Posted by Csaba Gabor on May 26 2004 8:25am [Delete] [Edit]

you have a table where each row is associated with a certain group (For
example, orders are associated with the customers placing them) where
each item WITHIN the group has a distinct number (For example, each
person my have a sequence of competition results – each person,
therefore, has a 1st, 2nd, 3rd… competition).
If you would like to
renumber items within their group so that each has the same baseline
(say 0), here is an example way to proceed:

Name VARCHAR(31), GroupId VARCHAR(31), ValWithinGroup INTEGER);
INSERT INTO Groups VALUES (null, "Davy", "Boy", 2);
INSERT INTO Groups VALUES (null, "Mary", "Girl", 2);
INSERT INTO Groups VALUES (null, "Bill", "Boy", 5);
INSERT INTO Groups VALUES (null, "Jill", "Girl", -3);
INSERT INTO Groups VALUES (null, "Fred", "Boy", 3);

# Find the lowest value for each group
AS baseVal FROM Groups GROUP BY GroupId;
# create an index so mySQL can efficiently match
# finally, make the baseline adjustment
UPDATE Groups LEFT JOIN GroupSum USING (GroupId)
SET ValWithinGroup=ValWithinGroup-baseVal;
# 1 Davy Boy 0
# 2 Mary Girl 5
# 3 Bill Boy 3
# 4 Jill Girl 0
# 5 Fred Boy 1
#Each group ("Boy", "Girl") now has a (lowest) ValWithinGroup entry of 0.

That index addition is necessary because on larger tables mySQL would
rather die than figure to (internally) index a single column join.

I was not able, using mySQL 4.1.1, to do this as a subquery:
Groups LEFT JOIN (SELECT GroupId, MIN(ValWithinGroup) AS baseVal FROM
Groups GROUP BY GroupId) AS GrpSum USING (GroupId) SET

Csaba Gabor

Posted by Michał Łukaszewski on June 10 2004 9:23pm [Delete] [Edit]

UPDATE Syntax with "on-line" updating value limitations.

had a problem – a had to update a column "rate" but if the existince or
new value is greater then 5 this "5" will be finally value in field.
So, I do it in one "magick" query ;)
Here an example:

"3" is a some value, from form or something

update item
set rate = case when round((rate+3)/2) < 6 then round((rate+3)/2) else 5 end
where id = 1 and rate <= 6;


Posted by Alex de Landgraaf on July 21 2004 10:07pm [Delete] [Edit]

sometimes run into the problem that you want to replace a substring
occuring in a column with a different string, without touching the rest
of the string. The solution is surprisingly simple, thanks to MySQL:

UPDATE xoops_bb_posts_text
SET post_text=(
REPLACE (post_text,

the string function REPLACE, all items in the post_text column with
'' get this substring replaced by
''. Ideal when writing a script is just too much effort.

Posted by Justin Swanhart on July 29 2004 5:32pm [Delete] [Edit]

you have a lot of processes that could be updating a column value in a
table. If you want to return the value before you updated it without
using a seperate select (which unless you lock the table could return a
different value than is updated) then you can use a mysql variable like

update some_table
set col = col + 1
where key = 'some_key_value'
and @value := col

The @value := col will always evaluate to true and will store the col value before the update in the @value variable.

You could then do

select @value;

in order to see what the value was before you updated it

Posted by Vladimir Petrov on December 9 2004 1:44pm [Delete] [Edit]

MySQL uses Watcom (Oracle) syntax for UPDATE, so it's possible to write something like:

update Table1 t1
join Table2 t2 on t1.ID=t2.t1ID
join Table3 t3 on t2.ID=t3.t2ID
set t1.Value=12345
where t3.ID=54321

Posted by Stuart Colville on January 18 2005 10:50am [Delete] [Edit]

you want to merge two columns together into one , for example, to join
together firstnames and surnames into a column called "name". Just use
CONCAT. Without the WHERE clause the UPDATE creates the name entry for
every record in the table.

UPDATE table_name SET name = CONCAT(forename, ' ', surname)

Posted by Matt Ryan on February 16 2005 8:20pm [Delete] [Edit]

Here's a workaround for the update/subquery/cant do self table "bug"

Senario is, ID 8 has multiple records, only the last (highest) record needs to be changed

update t1 set c1 = 'NO'
where id='8'
order by recno desc limit 1

I would prefer update t1 set c1='NO' WHERE ID=8 AND RECNO = (SELECT MAX(RECNO) FROM T1 WHERE ID=8)

But that's not currently allowed

Posted by Bob Terrell on February 24 2005 4:34am [Delete] [Edit]

you want to update a table based on an aggregate function applied to
another table, you can use a correlated subquery, for example:

UPDATE table1 SET table1field = (SELECT MAX(table2.table2field) FROM table2 WHERE table1.table1field = table2.table2field)

can be helpful if you need to create a temporary table storing an ID
(for, say, a person) and a "last date" and already have another table
storing all dates (for example, all dates of that person's orders).

Additional information on MySQL correlated subqueries is at

Posted by Ken Miller on April 6 2005 6:34am [Delete] [Edit]

I was looking at this example:

update item
set rate = case when round((rate+3)/2) < 6 then round((rate+3)/2) else 5 end
where id = 1 and rate <= 6;

I think it can be done simpler with LEAST:

update item
set rate = least(round((rate+3)/2), 5)
where id = 1 and rate <= 6;

(I could be wrong, but that looks like it ought to work.)

Posted by David Friedman on May 5 2005 8:58pm [Delete] [Edit]

The UPDATE can apparently be used to implement a semaphore (pardon my pseudocode):

while TRUE {
..UPDATE table SET value = 1
….WHERE value = 0 and name = 'name'
..if no. of rows affected > 0, break
..else wait and try again

code above waits until the semaphore is "cleared" (value = 0) and then
"sets" it (value = 1). When done, you "clear" the semaphore by

UPDATE table SET value = 0 WHERE name = 'name'

assumption is that the UPDATE is "atomic" in that no concurrent access
by another process can occur between testing and setting the value

Posted by Mohamed Hossam on May 9 2005 2:38am [Delete] [Edit]

have posted this in the Flow Control Functions page last year but I
still see people asking how to update multiple rows. So, here it is

A very server resources friendly method to update
multiple rows in the same table is by using WHEN THEN (with a very
important note).

UPDATE tbl_name SET fld2 = CASE fld1
WHEN val1 THEN data1
WHEN val2 THEN data2

note is: do not forget ELSE. If you do not use it, all rows that are
outside the range of your updated values will be set to blank!

Posted by Christian Hansel on July 1 2005 9:13am [Delete] [Edit]

If you wish to use an increment based on subset of a table you may combine UPDATE with Variables:

A table that contains entries of different categories, in which an
internal order needs to represented ( lets say a table with busstops on
different routes). If you add new entries or move stops from one route
to another you will most likely want to increment the position of the
busstop within this route. That's how you can do it

table busstops

id | route | busstop | pos
1 | 1 | A | 1
2 | 1 | B | 2
3 | 1 | C | 3
4 | 2 | C | 1
5 | 2 | D | 2
6 | 2 | A | 3
7 | 2 | E | 4
8 | 2 | F | 5
9 | 2 | G | 6
10 | 2 | H | 7

Moving D,E,F,G To route 1
SET @pos=(SELECT max(t1.pos) FROM busstops t1 WHERE t1.route = 1 );
UPDATE busstops SET pos = ( SELECT @pos := @pos +1 ), route =1 WHERE id IN (5,7,8,9)

doubt this could be done otherwise since referencing the table you wish
to update within the subquery creates circular references

DELETE or UPDATE i.e. when a row of a subset is lost/deleted/moved away
from it, the whole subset will need to be reordered. This can be done
similarily :

SET @pos=0;
UPDATE busstops SET pos = ( SELECT @pos := @pos +1 ) WHERE route = 1 ORDER BY pos ASC

Chris H (chansel0049)

Posted by Anders Elton on November 24 2005 10:03pm [Delete] [Edit]

I experienced a weird issue converting from 4 to 5.

A is a normal table, B is a temporary table:
Worked in 4
update A, B set A.population=B.pop_count where

In version 5, however, the above query only updated one element while still matching "all"

In 5 I had to do it like this:
update A RIGHT JOIN B on set A.population=B.pop_count
Updates all population counts correctly.


Posted by Jan Slauer on December 9 2005 1:37pm [Delete] [Edit]

I had the same problem after update from mysql 4.x.x to 5.x.x. I just exported all the tables to files via PhpMyAdmin
and imported them back. Now everything works fine.

Posted by Sadder But Wiser on February 8 2006 4:11pm [Delete] [Edit]

Server Version: 5.0.10-beta-max-log
Engine: MyISAM

adding to Mike Zhange's comment above, the problem seems to occur in
single table update as well. Update's effects are reflected immediately
(as in intra-statement):

create table a (id int, v1 float, v2 float);
insert into a values(1,1,2);
update a SET v1=v1/(v1+v2), v2=v1/(v1+v2);
select * from a;

Actual Result: 1 0.333333 0.142857
Expected Result: 1 0.333333 0.333333

Posted by [name withheld] on March 17 2006 1:09pm [Delete] [Edit]

Related to the post of Mohamed Hossam on May 9 2005 4:38am
A more general method to updtate more one row:

UPDATE table SET f1='foo', f2=
WHERE f5='afected'

This set the values of field 'f2' according to the values of field 'f3' in the rows field f5 'afected'.

Posted by [name withheld] on March 21 2006 3:05am [Delete] [Edit]

RE: Sadder But Wiser on February 8 2006 5:11pm

create table a (id int, v1 float, v2 float);
insert into a values(1,1,2);
update a SET v1=v1/(v1+v2), v2=v1/(v1+v2);
select * from a;

Actual Result:* 1 0.333333 0.142857
Expected Result: 1 0.333333 0.333333

works exactly to the specifications of the documentation. the value of
v1 is changed to 0.333333 before the equation to update v2 is
evaluated, so the expected result is 0.142857

Posted by Rafi B. on April 26 2006 6:44pm [Delete] [Edit]

is a way to use multiple tables in your UPDATE statement, but actually
copying one row values into the other, meaning, we're using the same

UPDATE jobs AS toTable, jobs AS fromTable
toTable.job_type_id = fromTable.job_type_id,
toTable.job_company_id = fromTable.job_company_id,
toTable.job_source = fromTable.job_source,
(toTable.job_id = 6)
(fromTable.job_id = 1)

cool. What I'm doing here is copying the information I need from the
row where job_id=1 to the row where job_id=6, on the same table.

Posted by Christopher Marshall on June 7 2006 3:25pm [Delete] [Edit]

Boyle's commment above was just what I was trying to do, update one
table based on a relationship between that table and another. His
example was:

update t1,t2 set t1.field=t2.value where t1.this=t2.that;

That strikes me as an elegant syntax. Here is the closest I could come up with for doing that on Oracle:

update t1 set t1.field=(select value from t2 where t1.this=t2.that) where t1.this in (select that from t2);

That strikes me as convoluted by comparison.

Posted by venky kris on June 8 2006 2:06pm [Delete] [Edit]

Just following up on Matt Ryan's Post

Matt Ryan Writes :
>>Here's a workaround for the update/subquery/cant do self >>table "bug"

>>Senario is, ID 8 has multiple records, only the last
>>(highest) record needs to be changed

>>update t1 set c1 = 'NO'
>>where id='8'
>>order by recno desc limit 1

You can also accomplish the same by the following query :

update t1 , (select id ,max(recno) as recno from t1 where id=8 group by recno) tt
set t1.c1 = 'NO'
where and

Comments are welcome.

Posted by Paul Decowski on August 1 2006 9:43am [Delete] [Edit]

Regarding Justin Swanhart's comment about retrieving a field's value in UPDATE query.

> update some_table
> set col = col + 1
> where key = 'some_key_value'
> and @value := col

> The @value := col will always evaluate to true and will store the col value before the update in the @value variable.

fact, in won't if col is NULL (0, empty string etc.) – then the
condition is not met and the update query won't be processed. The
correct condition would be:

AND ((@value := col) OR (1 = 1))

It was very helpful to me anyway. Thx Justin!

Posted by [name withheld] on October 24 2006 9:40pm [Delete] [Edit]

To update a column of a table with a rank based on subsets of data, the IF() function does a wonderful job.

summary table (in this case created to hold summary counts of other
genealogy data, based on the two fields that make up the PRIMARY key)
often contains unique key fields and one or more summary totals (Cnt in
this case). Additional ranking fields in the summary table can be
easily updated to contain rankings of the Cnt field using the IF
function and
local variables.

Table DDL:

CREATE TABLE countsbyboth (
SurnameID int(11) unsigned NOT NULL default '0',
GedID int(11) unsigned NOT NULL default '0',
Cnt int(11) unsigned NOT NULL default '0',
sRank int(11) unsigned NOT NULL default '0',
nRank int(11) unsigned NOT NULL default '0',
KEY SurnameID (SurnameID,Cnt),
KEY GedID (GedID,Cnt)

After populating the table with rows containing key and summary data (and leaving the rank field(s) to be updated in
a subsequent step), the rank fields can be updated using syntax similar to the following:

update countsbyboth set srank=0, nrank=0;
set @rnk:=1, @gedid=0;
update countsbyboth
set srank=if(@gedid=(@gedid:=gedid), (@rnk:=@rnk+1),(@rnk:=1))
order by gedid desc, cnt desc;
set @rnk:=1, @snmid=0;
update countsbyboth
set nrank=if(@snmid=(@snmid:=surnameid), (@rnk:=@rnk+1),(@rnk:=1))
order by surnameid desc, cnt desc;

Query OK, 11752 rows affected (0.08 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 11752 rows affected (0.24 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 11752 rows affected (0.19 sec)

looks convoluted, but is really quite simple. The @rnk variable needs
to be initialized, and the keyval variable (in this case @gedid or
@snmid) needs to be set to a value that will not be matched by the
first record. The IF() function checks the previous key value (left
side) against the current key value (right side), and either increments
the @rnk variable when the desired key value is the same as the
previous records, or reset the @rnk variable to 1 when the key value

This can be easily extended to accomodate ranking on
more than one key value, and does not require sub-selects that take
considerable resources for a large table.

This example
intentionally assigns different ranks to equal values of Cnt for a
given key, to facilitate reporting where column headings contain the
rank value.

Posted by [name withheld] on October 24 2006 9:51pm [Delete] [Edit]

In the previous example, if the @rnk value is initialed with
set @rnk:=0
rather than
set @rnk:=1 ,
then it won't matter whether or not the first record's key value matches the 'key value' @gedid variable.

Posted by Jon Meredith on October 31 2006 5:09pm [Delete] [Edit]

Thanks for Justin Swanhart/Paul Decowski tip. As of 5.0.18 it looks like the optimiser has been improved so the

AND ((@value := col) OR (1 = 1))

gets optimised out as 'true' and @value is left as NULL after the update.

I got it to work again by rewriting as

update some_table
set col = col + 1
where key = 'some_key_value'
and ((@value := col) IS NULL OR (@value := col) IS NOT NULL)

you get a true value either way and value will get set. Be careful what
you put on the right-hand-side as it could get evaluated twice.

Posted by Dewey Gaedcke on December 27 2006 8:34pm [Delete] [Edit]

Above in the docs, it says "you cannot update a table and select from the same table in a subquery"

This is true but there are two simple ways around this limit.
1) nest the subquery 2 deep so it is fully materialized before the update runs. For example:
Update t1 set v1 = t3.v1 where id in
(select, t2.v1 from (select id, v1 from t1) t2) t3

2) use a self join rather than a subquery

Posted by Lars Aronsson on March 9 2007 10:07pm [Delete] [Edit]

databases has a keyword NOWAIT that can be used with UPDATE, causing
the update to abort if it would get stuck waiting for locks. This
keyword is not available in MySQL. Just letting you know, so you can
stop looking for it.

Posted by John Batzel on March 16 2007 1:58pm [Delete] [Edit]

UPDATE 'bug' mentioned above is apparently related to upgrading from
4.x to 5.0x. The indexes are slightly different formats, and it breaks
*some* things. myisamchk/check table won't fix this. Dropping and
re-adding the indexes will. (And dumping the table to file and
reloading it is just recreating the indexes with lots more IO than you
need to do.)

Posted by James Goatcher on March 16 2007 9:28pm [Delete] [Edit]

This example/tip/bug-report uses MySQL version 5.0.19.

updating one table using values obtained from another table, the manual
describes the "update table1, table2" syntax, but does not delve into
the correlated subquery approach very much. It also does not point out
a VERY important execution difference.

Consider the following script:
drop table if exists test_1;
drop table if exists test_2;

col_pk integer NOT NULL,
col_test integer

alter table test_1 add PRIMARY KEY (col_pk);

col_pk_join integer NOT NULL,
col_test_new integer

insert into test_1 (col_pk, col_test) values ( 1, null );
insert into test_1 (col_pk, col_test) values ( 2, null );

insert into test_2 (col_pk_join, col_test_new) values ( 1, 23 );
insert into test_2 (col_pk_join, col_test_new) values ( 1, 34 );
insert into test_2 (col_pk_join, col_test_new) values ( 2, 45 );

select * from test_1;
select * from test_2;

# This update should NOT work, but it does.
UPDATE test_1 t,
test_2 tmp
set t.col_test = tmp.col_test_new
where t.col_pk = tmp.col_pk_join;

select * from test_1;

The output of the select and update statements is:

2 rows in set

3 rows in set

Query OK, 2 rows affected
Rows matched: 2 Changed: 2 Warnings: 0

Query OK, 0 rows affected

2 rows in set

that the update did NOT produce any errors or warnings. It should have.
Why? Because a join on value 1 produces two values from table test_2.
Two values cannot fit into a space for one. What MySQL does in this
case is use the first value and ignore the second value. This is really
bad in my opinion because it is, in essence, putting incorrect data
into table test_1.

Replace the update statement above with:
UPDATE test_1 t1
set t1.col_test = (
select col_test_new
from test_2 t2
where t1.col_pk = t2.col_pk_join

This will produce the appropriate error for the given data:
"ERROR 1242 : Subquery returns more than 1 row"
and will not perform any update at all, which is good (it protects table test_1 from getting bad data).

if you have different data……..if you comment out one of the "1"
values inserted into table test_2 and use the correlated subquery
update instead of the multi-table update, table test_1 will get updated
with exactly what you expect.

The moral of this
example/tip/bug-report: do not use the multi-table update. Use the
correlated subquery update instead. It's safe. If you keep getting an
error when you think you shouldn't, you either have bad data in your
source table or you need to rework your subquery such that it produces
a guaranteed one-row result for each destination row being updated.

reason I call the multi-table update a bug is simply because I feel it
should produce the same or similar error as the correlated subquery
update. My hope is that MySQL AB will agree with me.

Posted by Luciano Fantuzzi on March 23 2007 2:32am [Delete] [Edit]

Este sencillo script permite recrear el indice de una columna de forma automatica.
Si una columna tiene una restriccion NOT NULL, sera necesario usar
primero 'ALTER TABLE' para quitarle temporalmente la restriccion.

/* INICIO del script */

#En caso de tener con NOT NULL alguna columna (Ejemplo)

#Cambio todos los valores a NULL (para que no haya riesgo de valores duplicados con restricciones UNIQUE)
UPDATE MiTabla SET columna=NULL;

#Declaro una variable como contador (puede ser 1,2,3… o el num desde donde queremos empezar)
SET @c:=1;

UPDATE MiTabla SET columna=(SELECT @c:=@c+1);

#Ahora podemos usar ALTER TABLE nuevamente si queremos cambiar la columna a NOT NULL (en caso de que la hayamos cambiado)

/* FIN del script */

en cuenta de que los indices principales (los declarados como PRIMARY
KEY, por ejemplo, o los que se usan para linquear tablas) NO DEBERIAN
CAMBIARSE, ya que se estropearian los vinculos entre las tablas! Esto
podria evitarse declarando las claves foraneas (FOREIGN KEY) de las
tablas de linqueo con el valor ON UPDATE CASCADE (lo que al actualizar
los indices refrescaria los links entre las tablas).

Posted by rasu thangs on March 30 2007 6:13am [Delete] [Edit]

hope this could be much more helpful query to update a single (or)
multiple field with multiple values based on the conditions:

update sample_table set shift_1 = case when shift_1='A' then 'C' when shift_1='B' then 'A' when shift_1='C' then 'B' end

We'll come up with several enhanced queries in sooner..
For any explanations, feel free to contact us @


Posted by Amit Ruhela on April 27 2007 5:46pm [Delete] [Edit]

u want to update all columns of a table and if u want to do sth like in
"insert" command where u don't hav to specify the name of all columns ,
then how can u go for that?
it will be sth like this
Update book set values(1,2,3);

Posted by Marc Vos on July 18 2007 1:15pm [Delete] [Edit]

Here's the easy way to update a column in a table using values from other tables.

update db1.a, (
select distinct b.col1, b.col2
from db2.b, db2.c, db2.d
where b.col1<>" and d.idnr=b.idnr and c.user=d.user and c.role='S'
order by b.col1) as e
set a.col1 = e.col1
where a.idnr = e.col1

point is that every select statement returns a table. Name the result
and you can access its columns. In this example I called the result 'e'.

Posted by Richard Bronosky on September 5 2007 5:05pm [Delete] [Edit]

Vos led me to a solution to a problem that has been troubling me for a
long time. As a DBA I often have to support application developers who
need to have data I control presented in a specific manner. This always
results in a table based on their needs and populating the columns with
data from existing tables. Usually it something like 15 columns from
table A, 5 from table B, 30 from table c, and 230 from table d. In the
past I have done this with either a series of "create temporary table
t1 as select … join …" statements until I get the right set of

I never could figure out how to set the value of
multiple columns with nesting a select statement dedicated to each
column. Now I've got it. I'm attaching a transcript of doing it both
ways. The statements use the tables that already exist in the mysql
schema (at least in 5.0), so you can easily recreate this on your box
in a test schema.


Query OK, 0 rows affected (0.00 sec)

CREATE TABLE test (t_id INT,k_id INT, t_name CHAR(64), t_desc TEXT) AS
SELECT help_topic_id AS t_id, help_keyword_id AS k_id, NULL AS t_name, NULL AS t_desc FROM mysql.help_relation LIMIT 10

Query OK, 10 rows affected (0.01 sec)
Records: 10 Duplicates: 0 Warnings: 0


10 rows in set (0.00 sec)

## This is the elegant single select solution! ##
UPDATE test AS t, (SELECT * FROM mysql.help_topic) AS h SET,
WHERE t.t_id=h.help_topic_id

Query OK, 10 rows affected (0.04 sec)
Rows matched: 10 Changed: 10 Warnings: 0


10 rows in set (0.03 sec)


Query OK, 0 rows affected (0.00 sec)

CREATE TABLE test (t_id INT,k_id INT, t_name CHAR(64), t_desc TEXT) AS
SELECT help_topic_id AS t_id, help_keyword_id AS k_id, NULL AS t_name, NULL AS t_desc FROM mysql.help_relation LIMIT 10

Query OK, 10 rows affected (0.01 sec)
Records: 10 Duplicates: 0 Warnings: 0


10 rows in set (0.00 sec)

## This is the nasty one select for each column that needs to be updated method! ##
t.t_name=(SELECT name FROM mysql.help_topic WHERE t.t_id=help_topic_id),
t.t_desc=(SELECT substr(url,1-locate('/',reverse(url))) FROM mysql.help_topic WHERE t.t_id=help_topic_id)

Query OK, 10 rows affected (0.00 sec)
Rows matched: 10 Changed: 10 Warnings: 0


10 rows in set (0.00 sec)


Posted by Joris Kinable on April 30 2008 9:34am [Delete] [Edit]

multiple fields based on query results can be quite expensive if the
same query has to be executed multiple times. Imagine the following

summary(X,A,B,C,D) and a query which returns: (X,E,F) and
you want to update the summary table fields C and D with the values of
E and F:

Summary: (1,2,3,0,0),(10,12,13,0,0) and query result:
(1,4,5),(10,14,15) should result in the updated summary table:

BAD SOLUTION (same query is evaluated twice!):

UPDATE summary SET C=(SELECT E FROM (query) q WHERE summary.X=q.X), D=(SELECT F FROM (query) q WHERE summary.X=q.X)

GOOD SOLUTION (query is only evaluated once):

UPDATE summary AS t, (query) AS q SET t.C=q.E, t.D=q.F WHERE t.X=q.X

Posted by Nigel Smith on September 16 2008 9:31am [Delete] [Edit]

Example of updating a table using a group selection from another table:-
update tableA,
select idTableA,min(valueField) as minV from tableB group by idTableA
) as T
set tableA.minValue=minV where tableA.idTableA=T.idTableA

Posted by Roger Morris on September 23 2008 4:04am [Delete] [Edit]

To swap two values in a single table. If you need to keep the lower value in a certain column:
mysql> select * from test;

4 rows in set (0.00 sec)

### (@olditem1:=item1) will assign the value of item1 *before* the update.

mysql> update test set item1=item2,item2=@olditem1 where (@olditem1:=item1) and item1>item2;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0

mysql> select * from test;

4 rows in set (0.00 sec)

mysql 3년쓰면서 업데이트문도 헷갈린다 -_-

MYSQL 데이터 타입

분류 데이터 타입 범위 저장소크기
정수 Bit O 또는 1 bit
Int -2,147,483,648 ~ 2,147,483,647 4 바이트
Smallint -32,768 ~ 32,767 2 바이트
Tinyint 0 ~ 255 1 바이트
Bigint -2^63 ~ 2^63-1 8 바이트
부동소수점 Float[n] -1.79E+308 ~ 1.79E+308
n = 1~24
4 바이트
Float[n] -1.79E+308 ~ 1.79E+308
n = 25~53
8 바이트
Real -3.40E + 38 ~ 3.40E + 38 4 바이트
문자데이터 char[n] n = 1~8000 n 바이트
Varchar[n] n = 1~8000 입력한 데이터의 길이
Text 최대 2,147,483,647자의 가변길이  
Nchar n = 1~4000 n*2 바이트
nvarchar n = 1~4000 입력한 데이터의 길이*2 바이트
Ntext 최대 1,073,741,823자의 가변길이  
이진데이터 binary n = 1~8000 n+4 바이트
varbinary n = 1~8000 입력한 데이터의 길이+4 바이트
Image 최대 2,147,483,647자의 가변길이  
날짜와시간 datetime 1753/1/1~9999/12/31 8 바이트
smalldatetime 1900/1/1~2079/6/6 4 바이트
화폐 money -922,337,203,685,477.5808~ +922,337,203,685,477.5807 8 바이트
smallmoney -214,748.3648~214,748.3647 4 바이트

MYSQL 언어설정


init-connect = set names euckr

init-connect = set names euckr

쿼리로 설정
set names euc-kr

대소문자 구별한다 -_-