SQLServer中exists和except用法

一、exists

1.1 说明

EXISTS(包括 NOT EXISTS)子句的返回值是一个BOOL值。EXISTS内部有一个子查询语句(SELECT ... FROM...)，我将其称为EXIST的内查询语句。其内查询语句返回一个结果集。EXISTS子句根据其内查询语句的结果集空或者非空，返回一个布尔值。Link

exists：强调的是是否返回结果集，不要求知道返回什么，比如：select name from student where sex = 'm' and mark exists(select 1 from grade where ...) ，只要exists引导的子句有结果集返回，那么exists这个条件就算成立了，大家注意返回的字段始终为1，如果改成"select 2 from grade where ..."，那么返回的字段就是2，这个数字没有意义。所以exists子句不在乎返回什么，而是在乎是不是有结果集返回。EXISTS = IN，意思相同不过语法上有点点区别，好像使用IN效率要差点，应该是不会执行索引的原因。Link

相对于inner join，exists性能要好一些，当她找到第一个符合条件的记录时，就会立即停止搜索返回TRUE。

1.2 示例

--EXISTS--SQL:select name from family_memberwhere group_level > 0and exists(select 1 from family_grade where family_member.name = family_grade.nameand grade > 90)--result:namecherrie--NOT EXISTS--SQL:select name from family_memberwhere group_level > 0and not exists(select 1 from family_grade where family_member.name = family_grade.nameand grade > 90)--result:namemazeyrabbit

二、except

2.1 说明

查询结果上EXCEPT = NOT EXISTS，INTERSECT = EXISTS，但是EXCEPT / INTERSECT的「查询开销」会比NOT EXISTS / EXISTS大很多。

except自动去重复，not in / not exists不会。

2.2 示例

--except--SQL:select name from family_memberwhere group_level > 0except(select name from family_grade)--result:namerabbit--NOT EXISTS--SQL:select name from family_memberwhere group_level > 0and not exists(select name from family_grade where family_member.name = family_grade.name)--result:namerabbitrabbit

三、测试数据

-- ------------------------------ Table structure for family_grade-- ----------------------------DROP TABLE [mazeytop].[family_grade]GOCREATE TABLE [mazeytop].[family_grade] ([id] int NOT NULL ,[name] varchar(20) NULL ,[grade] int NULL )GO-- ------------------------------ Records of family_grade-- ----------------------------INSERT INTO [mazeytop].[family_grade] ([id], [name], [grade]) VALUES (N'1', N'mazey', N'70')GOGOINSERT INTO [mazeytop].[family_grade] ([id], [name], [grade]) VALUES (N'2', N'cherrie', N'93')GOGO-- ------------------------------ Table structure for family_member-- ----------------------------DROP TABLE [mazeytop].[family_member]GOCREATE TABLE [mazeytop].[family_member] ([id] int NOT NULL ,[name] varchar(20) NULL ,[sex] varchar(20) NULL ,[age] int NULL ,[group_level] int NULL )GO-- ------------------------------ Records of family_member-- ----------------------------INSERT INTO [mazeytop].[family_member] ([id], [name], [sex], [age], [group_level]) VALUES (N'1', N'mazey', N'male', N'23', N'1')GOGOINSERT INTO [mazeytop].[family_member] ([id], [name], [sex], [age], [group_level]) VALUES (N'2', N'cherrie', N'female', N'22', N'2')GOGOINSERT INTO [mazeytop].[family_member] ([id], [name], [sex], [age], [group_level]) VALUES (N'3', N'rabbit', N'female', N'15', N'3')GOGOINSERT INTO [mazeytop].[family_member] ([id], [name], [sex], [age], [group_level]) VALUES (N'4', N'rabbit', N'female', N'15', N'3')GOGO-- ------------------------------ Table structure for family_part-- ----------------------------DROP TABLE [mazeytop].[family_part]GOCREATE TABLE [mazeytop].[family_part] ([id] int NOT NULL ,[group] int NULL ,[group_name] varchar(20) NULL )GO-- ------------------------------ Records of family_part-- ----------------------------INSERT INTO [mazeytop].[family_part] ([id], [group], [group_name]) VALUES (N'1', N'1', N'父亲')GOGOINSERT INTO [mazeytop].[family_part] ([id], [group], [group_name]) VALUES (N'2', N'2', N'母亲')GOGOINSERT INTO [mazeytop].[family_part] ([id], [group], [group_name]) VALUES (N'3', N'3', N'女儿')GOGO-- ------------------------------ Indexes structure for table family_grade-- ------------------------------ ------------------------------ Primary Key structure for table family_grade-- ----------------------------ALTER TABLE [mazeytop].[family_grade] ADD PRIMARY KEY ([id])GO-- ------------------------------ Indexes structure for table family_member-- ------------------------------ ------------------------------ Primary Key structure for table family_member-- ----------------------------ALTER TABLE [mazeytop].[family_member] ADD PRIMARY KEY ([id])GO-- ------------------------------ Indexes structure for table family_part-- ------------------------------ ------------------------------ Primary Key structure for table family_part-- ----------------------------ALTER TABLE [mazeytop].[family_part] ADD PRIMARY KEY ([id])GO

SQLServer中exists和except用法