I was wondering if it's possible to conditionally run certain statements in MySQL. Something like this:
IF EXISTS (SELECT * FROM information_schema.columns WHERE TABLE_NAME = 'test_table' AND COLUMN_NAME = 'userid' AND IS_NULLABLE = 'NO')
THEN
ALTER TABLE test_table MODIFY userid INT(11) NULL;
END IF;
I've done some googleing and I'm not pulling up anything useful or sane. Wondering if I'm missing something or if this is just a serious MySQL limitation.
Thanks to William_Wilson, Here's my working query. I still think this is totally backwards, but I supposed that's how MySQL rolls...
delimiter //
create procedure update_stuff()
begin
IF EXISTS (SELECT * FROM information_schema.columns WHERE TABLE_NAME = 'test_table' AND COLUMN_NAME = 'userid' AND IS_NULLABLE = 'NO')
THEN
ALTER TABLE test_table MODIFY userid INT(11) NULL;
END IF;
END
//
delimiter ;
-- Execute the procedure
call update_stuff();
-- Drop the procedure
drop procedure update_stuff;
I need to dynamically alter set of tables
Currently i can alter only one using below SP.
DROP PROCEDURE IF EXISTS add_version_to_actor;
DELIMITER $$
CREATE DEFINER=CURRENT_USER PROCEDURE add_version_to_actor (IN table_name VARCHAR(40) )
BEGIN
DECLARE colName TEXT;
SELECT column_name INTO colName
FROM information_schema.columns
WHERE table_schema = 'database_name'
AND table_name = table_name
AND column_name = 'column';
IF colName is null THEN
Alter Table database_name.table_name ADD `sequence` INT( 10 ) NOT NULL;
END IF;
END$$
DELIMITER ;
CALL add_version_to_actor('table1');
DROP PROCEDURE add_version_to_actor;
Currently 'table1' is a single parameter. I need to pass multiple table names as string like this CALL add_version_to_actor('table1','table2');and loop through all of them.How can this be achieved.
Thanks in advance.
According to mysql website i should be able to start a query with an if statement.
IF search_condition THEN statement_list
[ELSEIF search_condition THEN statement_list] ...
[ELSE statement_list]
END IF
but when i try this query
if (count(1)
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
WHERE TABLE_SCHEMA = 'dbname'
AND TABLE_NAME='tblname'
AND CONSTRAINT_NAME = 'con_name')
then
alter table table drop foreign key constraint_name;
end if
i get a syntax error saying i have the wrong syntax near "IF" and mysql workbench highlights the if saying syntax error, unexpected if.
i've tried with begin if, and omitting both begin and end if but the error is always the same.
You can't use if or while conditions out side a statement unless they are enclosed in a code block of begin - end. Hence db engine threw an error on your statement.
For your statement to work, you need a stored procedure and some changes to the statement as well.
Example:
delimiter //
drop procedure if exists drop_constraint //
create procedure drop_constraint(
in dbName varchar(64),
in tableName varchar(64),
in constraintName varchar(64) )
begin
declare cnt int default 0;
select count(1) into cnt
from INFORMATION_SCHEMA.TABLE_CONSTRAINTS
where
table_schema = dbName
and table_name = tableName
and constraint_name = constraintName;
-- now check if any found
if ( cnt > 0 ) then -- if found some
-- now, execute your alter statement
-- include your alter table statement here
end if;
end;
//
delimiter ;
Using the above procedure you can check and drop a constraint.
mysql> call drop_constraint( 'test', 'my_table', 'fk_name' );
You cannot, if both table have same structure (Or you put same structure instead of *) you can do this using unions this way
select * from sometable WHERE (SELECT 1 FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE TABLE_SCHEMA = 'dbname' AND TABLE_NAME='tblname' AND CONSTRAINT_NAME = 'con_name') = 1
union all
select * from anothertable WHERE (SELECT 1 FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE TABLE_SCHEMA = 'dbname' AND TABLE_NAME='tblname' AND CONSTRAINT_NAME = 'con_name') IS NULL
Alternatievly you can achieve this by using stored procedure (With almost same synbtax as you had wrote)
Here is my procedure code. I am trying to update Username by putting old username in where clause.but it doesn't work.
DELIMITER $$
DROP PROCEDURE IF EXISTS `databasename`.`UpdateUsername` $$
CREATE DEFINER=`root`#`localhost` PROCEDURE `UpdateUsername`
(IN uname VARCHAR(30),tid VARCHAR(100),username VARCHAR(30) )
BEGIN
UPDATE table_name SET Username=username WHERE Username=uname;
END $$
DELIMITER ;
please help me to FIX this problem.
Try to remove 'username' from line number '6' and use some other parameter name. It might be conflicting with your table Username field.
For example: UPDATE table_name SET Username=OTHER_PARAMETER_NAME WHERE Username=uname;
If tid is the table name, shouldn't you use tid instead of table_name in the update query?
Hi i have faced same issue give an alise to table name and try running update query
try this query
UPDATE table_name tn SET tn.Username=username WHERE tn.Username=uname;
the query i have used is :
update file_structure fs set fs.active_status = 'N' where fs.fileid = temp_fileid and fs.appid = temp_appid;
Is there a way to check if a column exists in a mySQL DB prior to (or as) the ALTER TABLE ADD coumn_name statement runs? Sort of an IF column DOES NOT EXIST ALTER TABLE thing.
I've tried ALTER IGNORE TABLE my_table ADD my_column but this still throws the error if the column I'm adding already exists.
EDIT: use case is to upgrade a table in an already installed web app-- so to keep things simple, I want to make sure the columns I need exist, and if they don't, add them using ALTER TABLE
Since mysql control statements (e.g. "IF") only work in stored procedures, a temporary one can be created and executed:
DROP PROCEDURE IF EXISTS add_version_to_actor;
DELIMITER $$
CREATE DEFINER=CURRENT_USER PROCEDURE add_version_to_actor ( )
BEGIN
DECLARE colName TEXT;
SELECT column_name INTO colName
FROM information_schema.columns
WHERE table_schema = 'connjur'
AND table_name = 'actor'
AND column_name = 'version';
IF colName is null THEN
ALTER TABLE actor ADD version TINYINT NOT NULL DEFAULT '1' COMMENT 'code version of actor when stored';
END IF;
END$$
DELIMITER ;
CALL add_version_to_actor;
DROP PROCEDURE add_version_to_actor;
Do you think you can try this?:
SELECT IFNULL(column_name, '') INTO #colName
FROM information_schema.columns
WHERE table_name = 'my_table'
AND column_name = 'my_column';
IF #colName = '' THEN
-- ALTER COMMAND GOES HERE --
END IF;
It's no one-liner, but can you at least see if it will work for you? At least while waiting for a better solution..
Utility functions and procedures
First, I have a set of utility functions and procedures that I use to do things like drop foreign keys, normal keys and columns. I just leave them in the database so I can use them as needed.
Here they are.
delimiter $$
create function column_exists(ptable text, pcolumn text)
returns bool
reads sql data
begin
declare result bool;
select
count(*)
into
result
from
information_schema.columns
where
`table_schema` = 'my_database' and
`table_name` = ptable and
`column_name` = pcolumn;
return result;
end $$
create function constraint_exists(ptable text, pconstraint text)
returns bool
reads sql data
begin
declare result bool;
select
count(*)
into
result
from
information_schema.table_constraints
where
`constraint_schema` = 'my_database' and
`table_schema` = 'my_database' and
`table_name` = ptable and
`constraint_name` = pconstraint;
return result;
end $$
create procedure drop_fk_if_exists(ptable text, pconstraint text)
begin
if constraint_exists(ptable, pconstraint) then
set #stat = concat('alter table ', ptable, ' drop foreign key ', pconstraint);
prepare pstat from #stat;
execute pstat;
end if;
end $$
create procedure drop_key_if_exists(ptable text, pconstraint text)
begin
if constraint_exists(ptable, pconstraint) then
set #stat = concat('alter table ', ptable, ' drop key ', pconstraint);
prepare pstat from #stat;
execute pstat;
end if;
end $$
create procedure drop_column_if_exists(ptable text, pcolumn text)
begin
if column_exists(ptable, pcolumn) then
set #stat = concat('alter table ', ptable, ' drop column ', pcolumn);
prepare pstat from #stat;
execute pstat;
end if;
end $$
delimiter ;
Dropping constraints and columns using the utilities above
With those in place, it is pretty easy to use them to check columns and constraints for existence:
-- Drop service.component_id
call drop_fk_if_exists('service', 'fk_service_1');
call drop_key_if_exists('service', 'component_id');
call drop_column_if_exists('service', 'component_id');
-- Drop commit.component_id
call drop_fk_if_exists('commit', 'commit_ibfk_1');
call drop_key_if_exists('commit', 'commit_idx1');
call drop_column_if_exists('commit', 'component_id');
-- Drop component.application_id
call drop_fk_if_exists('component', 'fk_component_1');
call drop_key_if_exists('component', 'application_id');
call drop_column_if_exists('component', 'application_id');
Make a count sentence with the example below by John Watson.
SELECT count(*) FROM information_schema.COLUMNS
WHERE COLUMN_NAME = '...'
and TABLE_NAME = '...'
and TABLE_SCHEMA = '...'
Save that result in an integer and then make it a condition to apply the ADD COLUMN sentence.
You can test if a column exists with:
IF EXISTS (
SELECT * FROM information_schema.COLUMNS
WHERE COLUMN_NAME = '...'
and TABLE_NAME = '...'
and TABLE_SCHEMA = '...')
...
Just fill in your column name, table name, and database name.
Although its quite an old post but still i feel good about sharing my solution to this issue. If column doesn't exist then an exception would occur definitely and then i am creating the column in table.
I just used the code below:
try
{
DATABASE_QUERY="SELECT gender from USER;";
db.rawQuery(DATABASE_QUERY, null);
}
catch (Exception e)
{
e.printStackTrace();
DATABASE_UPGRADE="alter table USER ADD COLUMN gender VARCHAR(10) DEFAULT 0;";
db.execSQL(DATABASE_UPGRADE);
}
You can create a procedure with a CONTINUE handler in case the column exists (please note this code doesn't work in PHPMyAdmin):
DROP PROCEDURE IF EXISTS foo;
CREATE PROCEDURE foo() BEGIN
DECLARE CONTINUE HANDLER FOR 1060 BEGIN END;
ALTER TABLE `tableName` ADD `columnName` int(10) NULL AFTER `otherColumn`;
END;
CALL foo();
DROP PROCEDURE foo;
This code should not raise any error in case the column already exists. It will just do nothing and carry on executing the rest of the SQL.
DELIMITER $$
DROP PROCEDURE IF EXISTS `addcol` $$
CREATE DEFINER=`admin`#`localhost` PROCEDURE `addcol`(tbn varchar(45), cn varchar(45), ct varchar(45))
BEGIN
#tbn: table name, cn: column name, ct: column type
DECLARE CONTINUE HANDLER FOR 1060 BEGIN END;
set cn = REPLACE(cn, ' ','_');
set #a = '';
set #a = CONCAT("ALTER TABLE `", tbn ,"` ADD column `", cn ,"` ", ct);
PREPARE stmt FROM #a;
EXECUTE stmt;
END $$
DELIMITER ;
This syntax work for me :
SHOW COLUMNS FROM < tablename > LIKE '< columnName >'
More in this post :
https://mzulkamal.com/blog/mysql-5-7-check-if-column-exist?viewmode=0
As per MYSQL Community:
IGNORE is a MySQL extension to standard SQL. It controls how ALTER TABLE works if there are duplicates on unique keys in the new table or if warnings occur when strict mode is enabled. If IGNORE is not specified, the copy is aborted and rolled back if duplicate-key errors occur. If IGNORE is specified, only one row is used of rows with duplicates on a unique key. The other conflicting rows are deleted. Incorrect values are truncated to the closest matching acceptable value.
So a working Code is:
ALTER IGNORE TABLE CLIENTS ADD CLIENT_NOTES TEXT DEFAULT NULL;
Data posted here:
http://dev.mysql.com/doc/refman/5.1/en/alter-table.html