如何在mysql中存在一列？

Question

ALTER TABLE user DROP COLUMN registered_date 

我們可以使用上述命令刪除一列。但是，我只能在列registered_date存在時才能刪除它？

Answer 1

IF EXISTS(SELECT * 
      FROM information_schema.COLUMNS 
      WHERE 
      TABLE_SCHEMA = 'db_name' 
      AND TABLE_NAME = 'table_name' 
      AND COLUMN_NAME = 'column_name') 
BEGIN 
ALTER TABLE table_name DROP COLUMN column_name 
END 

OR

IF (SELECT COUNT(*) 
      FROM information_schema.COLUMNS 
      WHERE 
      TABLE_SCHEMA = 'db_name' 
      AND TABLE_NAME = 'table_name' 
      AND COLUMN_NAME = 'column_name'>0) 
BEGIN 
ALTER TABLE table_name DROP COLUMN column_name 
END