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check if column exists before ALTER TABLE — mysql

Posted by: admin November 29, 2017 Leave a comment

Questions:

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

Answers:

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..

Questions:
Answers:

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;

Questions:
Answers:

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');

Questions:
Answers:

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.

Questions:
Answers:

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);
   } 

Questions:
Answers:

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.

Questions:
Answers:
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 ;

Questions:
Answers:

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.

Questions:
Answers:

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

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