MySQL 添加列，如果不存在

Question

IF NOT EXISTS (SELECT * FROM INFORMATION_SCHEMA.COLUMNS
  WHERE TABLE_NAME = 'email_subscription' AND COLUMN_NAME = 'subscribe_all')
  THEN 
  ALTER TABLE email_subscription
  ADD COLUMN subscribe_all TINYINT(1) DEFAULT 1,
  ADD COLUMN subscribe_category varchar(512) DEFAULT NULL;

我查看了大量示例。但是这个查询不起作用，我得到了错误:

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'IF NOT EXISTS (SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME =' at line 1

Answer 1

如果您的主机没有授予您创建或运行过程的权限，我想我找到了另一种使用 PREPARE/EXECUTE 和查询架构的方法:

SET @s = (SELECT IF(
    (SELECT COUNT(*)
        FROM INFORMATION_SCHEMA.COLUMNS
        WHERE table_name = 'table_name'
        AND table_schema = DATABASE()
        AND column_name = 'col_name'
    ) > 0,
    "SELECT 1",
    "ALTER TABLE table_name ADD col_name VARCHAR(100)"
));

PREPARE stmt FROM @s;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;