SQL

mysql add column if not exists

ALTER TABLE test ADD COLUMN IF NOT EXISTS column_a VARCHAR(255);

add column if not exists mysql

-- ALTER TABLE tbl_name ADD COLUMN column_name column_definition 
--		[FIRST|AFTER existing_column];
ALTER TABLE office ADD COLUMN phone VARCHAR(200) DEFAULT '000' AFTER name;
ALTER TABLE office ADD COLUMN flag INT(1) FIRST;
ALTER TABLE office ADD COLUMN last_col INT(2);	-- Last column is default position
-- ↓ Test it (Fiddle)

add column if not exists mysql

SELECT count(*) INTO @EXIST
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = 'mydatabase'
  AND COLUMN_NAME = 'mycolumn'
  AND TABLE_NAME = 'mytable' LIMIT 1;

SET @query = IF (@exist <= 0, 'ALTER TABLE mydatabase.`mytable`  ADD COLUMN `mycolumn` MEDIUMTEXT NULL',
    'select ' COLUMN EXISTS' status');
PREPARE stmt FROM @query;
EXECUTE stmt;