上述SQL语句的含义是,从users表中删除所有不在blacklist表中的行。NOT EXISTS子句用来限定只保留在blacklist表中不存在的行。 解释 DELETE FROM:表示要删除哪张表的数据 WHERE NOT EXISTS:NOT EXISTS是一个谓词子句,该子句与WHERE一起使用,用于删除不在黑名单中的用户。
SELECTcolumn_nameFROMtable_nameWHEREcolumn_nameNOTIN(SELECTcolumn_nameFROManother_table) 注意,"NOT IN"操作符在使用时需要确保子查询的结果集不包含NULL值,否则可能导致不符合预期的结果。 "NOT EXISTS": "NOT EXISTS"操作符用于判断子查询的结果集是否为空,如果为空,则返回真(True)。它通常用于条件判断中,检...
Using theWHERE NOT EXISTSclause can be a powerful tool when working with databases. It allows us to extract data that is not present in another table. This can help us to better understand our data and make more informed decisions.
Finding Records in One table that Don't Exist in Another Table (the Not-In Query) Now assume that we ONLY want to find the customers who have not placed any orders. Just like in the first scenario, we will use a Left Outer Join to accomplish this. This time, however, we’ll add ...
ALTER TABLE modifies a table definition by altering, adding, or dropping columns and constraints. ALTER TABLE also reassigns and rebuilds partitions, or disables and enables constraints and triggers.
How we work with data is also immensely important — if our database is optimized well (check one of our previous articles for advice on how to do that), we should not have that manyINorNOT INclauses to begin with. Even if we do, our SQL client of choice is usually capable of ass...
SQL 复制 SELECT name FROM mytable WHERE name = 'Bill'; 键范围锁放置在与从 Ben 到Bing 的名称范围对应的索引项上,因为名称 Bill 将插入到这两个相邻的索引项之间。 RangeS-S 模式键范围锁放置在索引项 Bing 上。 这样可阻止其他任何事务在索引项 Bill 与Ben 之间插入值(例如 Bing)。
Test Value'; EXEC InsertUnitMeasure @UnitMeasureCode = 'XYZ', @Name = 'Test Value'; EXEC InsertUnitMeasure @UnitMeasureCode = 'ABC', @Name = 'Another Test Value';SELECT*FROM#MyTempTable;-- CleanupDELETEFROMProduction.UnitMeasureWHEREUnitMeasureCodeIN('ABC','XYZ');DROPTABLE#MyTempTable; ...
selectedfromanothertableortables.INSERTwithanONDUPLICATEKEYUPDATEclause enables existing rowstobe updatedifa rowtobe inserted would cause a duplicate valueinaUNIQUEindexorPRIMARYKEY.Foradditional information aboutINSERT...SELECTandINSERT...ONDUPLICATEKEYUPDATE, see[HELP INSERT SELECT],andhttp://dev.mysql.com...
ALTER TABLE では、列と制約を変更、追加、または削除して、テーブルの定義を変更します。 また、ALTER TABLE では、パーティションを再割り当ておよび再構築したり、制約とトリガーを無効化および有効化したりもします。