如何在執行特定查詢之前檢查mysql中的空值條件？

Question

我想在不同的5個數據庫中執行腳本。

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()); 

現在，我想在這裏檢查，如果@CONSTRAINT_NAME是NOT NULL則只有下面的聲明應被執行。可能嗎？

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，這是可能的標準IF聲明：

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 

詳情請參閱reference manual。