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_member
where group_level > 0
and exists(select 1 from family_grade where family_member.name = family_grade.name
and grade > 90)
--result:
name
cherrie
--NOT EXISTS
--SQL:
select name from family_member
where group_level > 0
and not exists(select 1 from family_grade where family_member.name = family_grade.name
and grade > 90)
--result:
name
mazey
rabbit
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_member
where group_level > 0
except(select name from family_grade)
--result:
name
rabbit
--NOT EXISTS
--SQL:
select name from family_member
where group_level > 0
and not exists(select name from family_grade where family_member.name = family_grade.name)
--result:
name
rabbit
rabbit
三、测试数据
-- ----------------------------
-- Table structure for family_grade
-- ----------------------------
DROP TABLE [mazeytop].[family_grade]
GO
CREATE 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')
GO
GO
INSERT INTO [mazeytop].[family_grade] ([id], [name], [grade]) VALUES (N'2', N'cherrie', N'93')
GO
GO -- ----------------------------
-- Table structure for family_member
-- ----------------------------
DROP TABLE [mazeytop].[family_member]
GO
CREATE 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')
GO
GO
INSERT INTO [mazeytop].[family_member] ([id], [name], [sex], [age], [group_level]) VALUES (N'2', N'cherrie', N'female', N'22', N'2')
GO
GO
INSERT INTO [mazeytop].[family_member] ([id], [name], [sex], [age], [group_level]) VALUES (N'3', N'rabbit', N'female', N'15', N'3')
GO
GO
INSERT INTO [mazeytop].[family_member] ([id], [name], [sex], [age], [group_level]) VALUES (N'4', N'rabbit', N'female', N'15', N'3')
GO
GO -- ----------------------------
-- Table structure for family_part
-- ----------------------------
DROP TABLE [mazeytop].[family_part]
GO
CREATE 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')
GO
GO
INSERT INTO [mazeytop].[family_part] ([id], [group], [group_name]) VALUES (N'2', N'2', N'母亲')
GO
GO
INSERT INTO [mazeytop].[family_part] ([id], [group], [group_name]) VALUES (N'3', N'3', N'女儿')
GO
GO -- ----------------------------
-- 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