如何在执行特定查询之前检查mysql中的空值条件？

Question

我想在不同的5个数据库中执行脚本。

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，如果@CONSTRAINT_NAME是NOT NULL则只有下面的声明应被执行。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，这是可能的标准IF声明：

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 

详情请参阅reference manual。