一、exists

1.1 說明

EXISTS(包括 NOT EXISTS)子句的返回值是一個BOOL值。EXISTS內部有一個子查詢語句(SELECT ... FROM...)，我將其稱為EXIST的內查詢語句。其內查詢語句返回一個結果集。EXISTS子句根據其內查詢語句的結果集空或者非空，返回一個布爾值。Link

exists：強調的是是否返回結果集，不要求知道返回什麽，比如：select name from student where sex = ‘m‘ and mark exists(select 1 from grade where ...) ，只要exists引導的子句有結果集返回，那麽exists這個條件就算成立了，大家註意返回的字段始終為1，如果改成“select 2 from grade where ...”，那麽返回的字段就是2，這個數字沒有意義。所以exists子句不在乎返回什麽，而是在乎是不是有結果集返回。EXISTS = IN，意思相同不過語法上有點點區別，好像使用IN效率要差點，應該是不會執行索引的原因。Link

相對於inner join，exists性能要好一些，當她找到第一個符合條件的記錄時，就會立即停止搜索返回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 說明

查詢結果上EXCEPT = NOT EXISTS，INTERSECT = EXISTS，但是EXCEPT / INTERSECT的「查詢開銷」會比NOT EXISTS / EXISTS大很多。

except自動去重復，not in / not exists不會。

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、測試數據

-- ----------------------------
-- 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‘父親‘)
GO
GO
INSERT INTO [mazeytop].[family_part] ([id], [group], [group_name]) VALUES (N‘2‘, N‘2‘, N‘母親‘)
GO
GO
INSERT INTO [mazeytop].[family_part] ([id], [group], [group_name]) VALUES (N‘3‘, N‘3‘, N‘女兒‘)
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

SQLServer中exists和except用法

本文出自 “不知不問” 博客，請務必保留此出處http://mazey.blog.51cto.com/12997993/1947482

SQLServer中exists和except用法