mysql - 如何在执行特定查询集之前检查 mysql 中的空条件？

Question

我正在尝试在不同的 5 个数据库中执行以下脚本。

SET @CONSTRAINT_NAME = (SELECT CONSTRAINT_NAME
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE REFERENCED_TABLE_NAME = 'StudentMaster'
AND TABLE_NAME = 'StudentSubjectMa'
AND CONSTRAINT_SCHEMA = DATABASE());

现在，我想在这里检查一下，如果 @CONSTRAINT_NAME 是 NOT NULL，那么应该只执行以下语句。可能吗？

    SET @DROP_CONSTRAINT_SQL = CONCAT("ALTER TABLE StudentSubjectMarksMap 
    DROP FOREIGN KEY `", @CONSTRAINT_NAME , "`");

    PREPARE DROP_CONSTRAINT_SQL_STMT FROM @DROP_CONSTRAINT_SQL;
    EXECUTE DROP_CONSTRAINT_SQL_STMT;
    DROP PREPARE DROP_CONSTRAINT_SQL_STMT;

Answer 1

是的，使用标准的 IF 语句是可能的:

IF @CONSTRAINT_NAME IS NOT NULL THEN 
    SET @DROP_CONSTRAINT_SQL = CONCAT("ALTER TABLE StudentSubjectMarksMap 
    DROP FOREIGN KEY `", @CONSTRAINT_NAME , "`");

    PREPARE DROP_CONSTRAINT_SQL_STMT FROM @DROP_CONSTRAINT_SQL;
    EXECUTE DROP_CONSTRAINT_SQL_STMT;
    DROP PREPARE DROP_CONSTRAINT_SQL_STMT;
END IF

参见 reference manual更多细节。