• 设为首页
  • 点击收藏
  • 手机版
    手机扫一扫访问
    迪恩网络手机版
  • 关注官方公众号
    微信扫一扫关注
    公众号
登陆 注册

15个初学者必看的基础SQL查询语句

原作者: [db:作者] 来自: [db:来源] 收藏 邀请

本文将分享15个初学者必看的基础SQL查询语句，都很基础，但是你不一定都会，所以好好看看吧。

1、创建表和数据插入SQL

我们在开始创建数据表和向表中插入演示数据之前，我想给大家解释一下实时数据表的设计理念，这样也许能帮助大家能更好的理解SQL查询。

在数据库设计中，有一条非常重要的规则就是要正确建立主键和外键的关系。

现在我们来创建几个餐厅订单管理的数据表，一共用到3张数据表，Item Master表、Order Master表和Order Detail表。

创建表：

创建Item Master表：

CREATE TABLE [dbo].[ItemMasters](
 [Item_Code] [varchar](20) NOT NULL,
 [Item_Name] [varchar](100) NOT NULL,
 [Price] Int NOT NULL,
 [TAX1] Int NOT NULL,
 [Discount] Int NOT NULL,
 [Description] [varchar](200) NOT NULL,
 [IN_DATE] [datetime] NOT NULL,
 [IN_USR_ID] [varchar](20) NOT NULL,
 [UP_DATE] [datetime] NOT NULL,
 [UP_USR_ID] [varchar](20) NOT NULL,
 CONSTRAINT [PK_ItemMasters] PRIMARY KEY CLUSTERED 
(
 [Item_Code] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

向Item Master表插入数据：

INSERT INTO [ItemMasters] ([Item_Code],[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE]
   ,[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('Item001','Coke',55,1,0,'Coke which need to be cold',GETDATE(),'SHANU'
   ,GETDATE(),'SHANU')

INSERT INTO [ItemMasters] ([Item_Code],[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE]
   ,[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('Item002','Coffee',40,0,2,'Coffe Might be Hot or Cold user choice',GETDATE(),'SHANU'
   ,GETDATE(),'SHANU')

INSERT INTO [ItemMasters] ([Item_Code],[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE]
   ,[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('Item003','Chiken Burger',125,2,5,'Spicy',GETDATE(),'SHANU'
   ,GETDATE(),'SHANU')

INSERT INTO [ItemMasters] ([Item_Code],[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE]
   ,[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('Item004','Potato Fry',15,0,0,'No Comments',GETDATE(),'SHANU'
   ,GETDATE(),'SHANU')

创建Order Master表：

CREATE TABLE [dbo].[OrderMasters](
 [Order_No] [varchar](20) NOT NULL,
 [Table_ID] [varchar](20) NOT NULL,
 [Description] [varchar](200) NOT NULL,
 [IN_DATE] [datetime] NOT NULL,
 [IN_USR_ID] [varchar](20) NOT NULL,
 [UP_DATE] [datetime] NOT NULL,
 [UP_USR_ID] [varchar](20) NOT NULL,
 CONSTRAINT [PK_OrderMasters] PRIMARY KEY CLUSTERED 
(
 [Order_No] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

向Order Master表插入数据：

INSERT INTO [OrderMasters]
   ([Order_No],[Table_ID] ,[Description],[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('Ord_001','T1','',GETDATE(),'SHANU' ,GETDATE(),'SHANU')

INSERT INTO [OrderMasters]
   ([Order_No],[Table_ID] ,[Description],[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('Ord_002','T2','',GETDATE(),'Mak' ,GETDATE(),'MAK')

INSERT INTO [OrderMasters]
   ([Order_No],[Table_ID] ,[Description],[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('Ord_003','T3','',GETDATE(),'RAJ' ,GETDATE(),'RAJ')

创建Order Detail表：

CREATE TABLE [dbo].[OrderDetails](
 [Order_Detail_No] [varchar](20) NOT NULL,
 [Order_No] [varchar](20) CONSTRAINT fk_OrderMasters FOREIGN KEY REFERENCES OrderMasters(Order_No),
 [Item_Code] [varchar](20) CONSTRAINT fk_ItemMasters FOREIGN KEY REFERENCES ItemMasters(Item_Code),
 [Notes] [varchar](200) NOT NULL,
 [QTY] INT NOT NULL,
 [IN_DATE] [datetime] NOT NULL,
 [IN_USR_ID] [varchar](20) NOT NULL,
 [UP_DATE] [datetime] NOT NULL,
 [UP_USR_ID] [varchar](20) NOT NULL,
 CONSTRAINT [PK_OrderDetails] PRIMARY KEY CLUSTERED 
(
 [Order_Detail_No] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

--Now let’s insert the 3 items for the above Order No 'Ord_001'.
INSERT INTO [OrderDetails]
   ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY]
   ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('OR_Dt_001','Ord_001','Item001','Need very Cold',3
   ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')

INSERT INTO [OrderDetails]
   ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY]
   ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('OR_Dt_002','Ord_001','Item004','very Hot ',2
   ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')

INSERT INTO [OrderDetails]
   ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY]
   ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('OR_Dt_003','Ord_001','Item003','Very Spicy',4
   ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')

向Order Detail表插入数据：

INSERT INTO [OrderDetails]
   ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY]
   ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('OR_Dt_004','Ord_002','Item002','Need very Hot',2
   ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')

INSERT INTO [OrderDetails]
   ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY]
   ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('OR_Dt_005','Ord_002','Item003','very Hot ',2
   ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')

INSERT INTO [OrderDetails]
   ([Order_Detail_No],[Order_No],[Item_Code],[Notes],[QTY]
   ,[IN_DATE],[IN_USR_ID],[UP_DATE],[UP_USR_ID])
  VALUES
   ('OR_Dt_006','Ord_003','Item003','Very Spicy',4
   ,GETDATE(),'SHANU' ,GETDATE(),'SHANU')

2、简单的Select查询语句

Select查询语句是SQL中最基本也是最重要的DML语句之一。那么什么是DML？DML全称Data Manipulation Language（数据操纵语言命令），它可以使用户能够查询数据库以及操作已有数据库中的数据。

下面我们在SQL Server中用select语句来查询我的姓名(Name)：

SELECT 'My Name Is SYED SHANU'
-- With Column Name using 'AS'
SELECT 'My Name Is SYED SHANU' as 'MY NAME'
-- With more then the one Column 
SELECT 'My Name' as 'Column1', 'Is' as 'Column2', 'SYED SHANU' as 'Column3'

在数据表中使用select查询：

-- To Display all the columns from the table we use * operator in select Statement.
Select * from ItemMasters
-- If we need to select only few fields from a table we can use the Column Name in Select Statement.
Select Item_Code
 ,Item_name as Item
 ,Price
 ,Description
 ,In_DATE
 FROM
 ItemMasters

3、合计和标量函数

合计函数和标量函数都是SQL Server的内置函数，我们可以在select查询语句中使用它们，比如Count(), Max(), Sum(), Upper(), lower(), Round()等等。下面我们用SQL代码来解释这些函数的用法：

select * from ItemMasters
-- Aggregate
-- COUNT() -> returns the Total no of records from table , AVG() returns the Average Value from Colum,MAX() Returns MaX Value from Column
-- ,MIN() returns Min Value from Column,SUM() sum of total from Column
Select Count(*) TotalRows,AVG(Price) AVGPrice
 ,MAX(Price) MAXPrice,MIN(Price) MinPrice,Sum(price) PriceTotal 
 FROM ItemMasters

-- Scalar 
-- UCASE() -> Convert to Upper Case ,LCASE() -> Convert to Lower Case,
-- SUBSTRING() ->Display selected char from column ->SUBSTRING(ColumnName,StartIndex,LenthofChartoDisplay)
--,LEN() -> lenth of column date,
-- ROUND() -> Which will round the value
SELECT UPPER(Item_NAME) Uppers,LOWER(Item_NAME) Lowers,
 SUBSTRING(Item_NAME,2,3) MidValue,LEN(Item_NAME) Lenths 
 ,SUBSTRING(Item_NAME,2,LEN(Item_NAME)) MidValuewithLenFunction, 
  ROUND(Price,0) as Rounded
 FROM ItemMasters

4、日期函数

在我们的项目数据表中基本都会使用到日期列，因此日期函数在项目中扮演着非常重要的角色。有时候我们对日期函数要非常的小心，它随时可以给你带来巨大的麻烦。在项目中，我们要选择合适的日期函数和日期格式，下面是一些SQL日期函数的例子：

-- GETDATE() -> to Display the Current Date and Time
-- Format() -> used to display our date in our requested format
Select GETDATE() CurrentDateTime, FORMAT(GETDATE(),'yyyy-MM-dd') AS DateFormats,
 FORMAT(GETDATE(),'HH-mm-ss')TimeFormats,
 CONVERT(VARCHAR(10),GETDATE(),10) Converts1,
 CONVERT(VARCHAR(24),GETDATE(),113),
 CONVERT(NVARCHAR, getdate(), 106) Converts2 ,-- here we used Convert Function 
 REPLACE(convert(NVARCHAR, getdate(), 106), ' ', '/') Formats-- Here we used replace and --convert functions.
 --first we convert the date to nvarchar and then we replace the '' with '/' 

select * from Itemmasters

Select ITEM_NAME,IN_DATE CurrentDateTime, FORMAT(IN_DATE,'yyyy-MM-dd') AS DateFormats,
 FORMAT(IN_DATE,'HH-mm-ss')TimeFormats,
 CONVERT(VARCHAR(10),IN_DATE,10) Converts1,
 CONVERT(VARCHAR(24),IN_DATE,113),
 convert(NVARCHAR, IN_DATE, 106) Converts2 ,-- here we used Convert Function 
 REPLACE(convert(NVARCHAR,IN_DATE, 106), ' ', '/') Formats
 FROM Itemmasters

DatePart –>  该函数可以获取年、月、日的信息。

DateADD –>  该函数可以对当前的日期进行加减。

DateDiff  –>  该函数可以比较2个日期。

--Datepart DATEPART(dateparttype,yourDate)
SELECT DATEPART(yyyy,getdate()) AS YEARs ,
DATEPART(mm,getdate()) AS MONTHS,
DATEPART(dd,getdate()) AS Days,
DATEPART(week,getdate()) AS weeks,
DATEPART(hour,getdate()) AS hours

--Days Add to add or subdtract date from a selected date.
SELECT GetDate()CurrentDate,DATEADD(day,12,getdate()) AS AddDays ,
 DATEADD(day,-4,getdate()) AS FourDaysBeforeDate 

 -- DATEDIFF() -> to display the Days between 2 dates
 select DATEDIFF(year,'2003-08-05',getdate()) yearDifferance , 
 DATEDIFF(day,DATEADD(day,-24,getdate()),getdate()) daysDifferent,
 DATEDIFF(month,getdate(),DATEADD(Month,6,getdate())) MonthDifferance

5、其他Select函数

Top —— 结合select语句，Top函数可以查询头几条和末几条的数据记录。

Order By —— 结合select语句，Order By可以让查询结果按某个字段正序和逆序输出数据记录。

--Top to Select Top first and last records using Select Statement.
Select * FROM ItemMasters
--> First Display top 2 Records
Select TOP 2 Item_Code
 ,Item_name as Item
 ,Price
 ,Description
 ,In_DATE
FROM ItemMasters
--> to Display the Last to Records we need to use the Order By Clause
-- order By to display Records in assending or desending order by the columns
Select TOP 2 Item_Code
 ,Item_name as Item
 ,Price
 ,Description
 ,In_DATE
FROM ItemMasters
ORDER BY Item_Code DESC

Distinct —— distinct关键字可以过滤重复的数据记录。

Select * FROM ItemMasters
--Distinct -> To avoid the Duplicate records we use the distinct in select statement
-- for example in this table we can see here we have the duplicate record 'Chiken Burger'
-- but with different Item_Code when i use the below select statement see what happen

Select Item_name as Item
 ,Price
 ,Description 
 ,IN_USR_ID 
 FROM ItemMasters
-- here we can see the Row No 3 and 5 have the duplicate record to avoid this we use the distinct Keyword in select statement.

select Distinct Item_name as Item
 ,Price
 ,Description 
 ,IN_USR_ID 
  FROM ItemMasters

6、Where子句

Where子句在SQL Select查询语句中非常重要，为什么要使用where子句？什么时候使用where子句？where子句是利用一些条件来过滤数据结果集。

下面我们从10000条数据记录中查询Order_No为某个值或者某个区间的数据记录，另外还有其他的条件。

Select * from ItemMasters
Select * from OrderDetails
--Where -> To display the data with certain conditions
-- Now below example which will display all the records which has Item_Name='Coke'
select * FROM ItemMasters WHERE ITEM_NAME='COKE'
-- If we want display all the records Iten_Name which Starts with 'C' then we use Like in where clause.
SELECT * FROM ItemMasters WHERE ITEM_NAME Like 'C%'

--> here we display the ItemMasters where the price will be greater then or equal to 40.
--> to use more then one condition we can Use And or Or operator.
--If we want to check the data between to date range then we can use Between Operator in Where Clause.
select Item_name as Item
 ,Price
 ,Description 
 ,IN_USR_ID 
 FROM ItemMasters
 WHERE
 ITEM_NAME Like 'C%' 
 AND 
 price >=40
--> here we display the OrderDetails where the Qty will be greater 3

Select * FROM OrderDetails WHERE qty>3

Where – In 子句

-- In clause -> used to display the data which is in the condition
select *
 FROM ItemMasters
 WHERE
 Item_name IN ('Coffee','Chiken Burger')

-- In clause with Order By - Here we display the in descending order.
select *
 FROM ItemMasters
 WHERE
 Item_name IN ('Coffee','Chiken Burger')
 ORDER BY Item_Code Desc

Where – Between子句

-- between -> Now if we want to display the data between to date range then we use betweeen keyword
select * FROM ItemMasters

select * FROM ItemMasters
 WHERE
 In_Date BETWEEN '2014-09-22 15:59:02.853' AND '2014-09-22 15:59:02.853'

select * FROM ItemMasters
 WHERE
 ITEM_NAME Like 'C%' 
 AND
 In_Date BETWEEN '2014-09-22 15:59:02.853' AND '2014-09-22 15:59:02.853'

查询某个条件区间的数据，我们常常使用between子句。

7、Group By 子句

Group By子句可以对查询的结果集按指定字段分组：

--Group By -> To display the data with group result.Here we can see we display all the AQggregate result by Item Name
Select ITEM_NAME,Count(*) TotalRows,AVG(Price) AVGPrice
 ,MAX(Price) MAXPrice,MIN(Price) MinPrice,Sum(price) PriceTotal 
 FROM
 ItemMasters
 GROUP BY ITEM_NAME

-- Here this group by will combine all the same Order_No result and make the total or each order_NO
Select Order_NO,Sum(QTy) as TotalQTY 
 FROM OrderDetails
 where qty>=2
 GROUP BY Order_NO

-- Here the Total will be created by order_No and Item_Code
Select Order_NO,Item_Code,Sum(QTy) as TotalQTY 
 FROM OrderDetails
 where qty>=2
 GROUP BY Order_NO,Item_Code
 Order By Order_NO Desc,Item_Code

Group By & Having 子句

--Group By Clause -- here this will display all the Order_no 
Select Order_NO,Sum(QTy) as TotalQTY 
 FROM OrderDetails
 GROUP BY Order_NO

-- Having Clause-- This will avoid the the sum(qty) less then 4 
Select Order_NO,Sum(QTy) as TotalQTY 
 FROM OrderDetails
 GROUP BY Order_NO
 HAVING Sum(QTy) >4

8、子查询

子查询一般出现在where内连接查询和嵌套查询中，select、update和delete语句中均可以使用。

--Sub Query -- Here we used the Sub query in where clause to get all the Item_Code where the price>40 now this sub 
--query reslut we used in our main query to filter all the records which Item_code from Subquery result
SELECT * FROM ItemMasters 
  WHERE Item_Code IN 
 (SELECT Item_Code FROM ItemMasters WHERE price > 40)

-- Sub Query with Insert Statement
INSERT INTO ItemMasters      ([Item_Code] ,[Item_Name],[Price],[TAX1],[Discount],[Description],[IN_DATE]
      ,[IN_USR_ID],[UP_DATE] ,[UP_USR_ID])
  Select 'Item006'
      ,Item_Name,Price+4,TAX1,Discount,Description
      ,GetDate(),'SHANU',GetDate(),'SHANU'
      from ItemMasters
      where Item_code='Item002'   

--After insert we can see the result as 
    Select * from ItemMasters

9、连接查询

到目前为止我们接触了不少单表的查询语句，现在我们来使用连接查询获取多个表的数据。

简单的join语句：

--Now we have used the simple join with out any condition this will display all the
-- records with duplicate data to avaoid this we see our next example with condition
SELECT * FROM Ordermasters,OrderDetails
-- Simple Join with Condition now here we can see the duplicate records now has been avoided by using the where checing with both table primaryKey field
SELECT * 
 FROM
 Ordermasters as M, OrderDetails as D
 where M.Order_NO=D.Order_NO
 and M.Order_NO='Ord_001'

-- Now to make more better understanding we need to select the need fields from both 
--table insted of displaying all column.
SELECT M.order_NO,M.Table_ID,D.Order_detail_no,Item_code,Notes,Qty
  FROM 
  Ordermasters as M, OrderDetails as D 
  where M.Order_NO=D.Order_NO         
 -- Now lets Join 3 table
 SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,
        I.Price*D.Qty as TotalPrice
  FROM 
  Ordermasters as M, OrderDetails as D,ItemMasters as I 
  where 
  M.Order_NO=D.Order_NO AND D.Item_Code=I.Item_Code

Inner Join,Left Outer Join,Right Outer Join and Full outer Join

下面是各种类型的连接查询代码：

--INNER JOIN 
--This will display the records which in both table Satisfy here i have used Like in where class which display the 
SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,I.Price*D.Qty as TotalPrice
 FROM 
 Ordermasters as M Inner JOIN OrderDetails as D 
 ON M.Order_NO=D.Order_NO
 INNER JOIN ItemMasters as I 
 ON  D.Item_Code=I.Item_Code
 WHERE
 M.Table_ID like 'T%'
--LEFT OUTER JOIN 
--This will display the records which Left side table Satisfy
 SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,I.Price*D.Qty as TotalPrice
 FROM 
 Ordermasters as M LEFT OUTER JOIN OrderDetails as D 
 ON M.Order_NO=D.Order_NO
 LEFT OUTER JOIN ItemMasters as I 
 ON  D.Item_Code=I.Item_Code
 WHERE
 M.Table_ID like 'T%'
--RIGHT OUTER JOIN 
--This will display the records which Left side table Satisfy
 SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,I.Price*D.Qty as TotalPrice
 FROM 
 Ordermasters as M RIGHT OUTER JOIN OrderDetails as D 
 ON M.Order_NO=D.Order_NO
 RIGHT OUTER JOIN ItemMasters as I 
 ON  D.Item_Code=I.Item_Code
 WHERE
 M.Table_ID like 'T%'

--FULL OUTER JOIN 
--This will display the records which Left side table Satisfy
 SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,I.Price*D.Qty as TotalPrice
 FROM 
 Ordermasters as M FULL OUTER JOIN OrderDetails as D 
 ON M.Order_NO=D.Order_NO
 FULL OUTER JOIN ItemMasters as I 
 ON  D.Item_Code=I.Item_Code
 WHERE
 M.Table_ID like 'T%'

10、Union合并查询

Union查询可以把多张表的数据合并起来，Union只会把唯一的数据查询出来，而Union ALL则会把重复的数据也查询出来。

Select column1,Colum2 from Table1
Union
Select Column1,Column2 from Table2

Select column1,Colum2 from Table1
Union All
Select Column1,Column2 from Table2

具体的例子如下：

--Select with different where condition which display the result as 2 Table result
select Item_Code,Item_Name,Price,Description FROM ItemMasters where price <=44
select Item_Code,Item_Name,Price,Description FROM ItemMasters where price >44

-- Union with same table but with different where condition now which result as one table which combine both the result.
select Item_Code,Item_Name,Price,Description FROM ItemMasters where price <=44
UNION
select Item_Code,Item_Name,Price,Description FROM ItemMasters where price >44

-- Union ALL with Join sample
 SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,I.Price*D.Qty as TotalPrice
 FROM 
 Ordermasters as M (NOLOCK)  Inner JOIN OrderDetails as D 
 ON M.Order_NO=D.Order_NO INNER JOIN ItemMasters as I 
 ON  D.Item_Code=I.Item_Code WHERE I.Price <=44 
Union ALL
 SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,I.Price*D.Qty as TotalPrice
 FROM 
 Ordermasters as M (NOLOCK)  Inner JOIN OrderDetails as D
 ON M.Order_NO=D.Order_NO INNER JOIN ItemMasters as I
 ON  D.Item_Code=I.Item_Code WHERE I.Price>44

11、公用表表达式（CTE）——With语句

CTE可以看作是一个临时的结果集，可以在接下来的一个SELECT,INSERT,UPDATE,DELETE,MERGE语句中被多次引用。使用公用表达式可以让语句更加清晰简练。

declare @sDate datetime,
    @eDate datetime;

select @sDate = getdate()-5,
    @eDate = getdate()+16;
--select @sDate StartDate,@eDate EndDate
;with cte as
  (
   select @sDate StartDate,'W'+convert(varchar(2),
      DATEPART( wk, @sDate))+'('+convert(varchar(2),@sDate,106)+')' as 'SDT'   
 union all
    select dateadd(DAY, 1, StartDate) ,
       'W'+convert(varchar(2),DATEPART( wk, StartDate))+'('+convert(varchar(2),
        dateadd(DAY, 1, StartDate),106)+')' as 'SDT'  
 FROM cte
 WHERE dateadd(DAY, 1, StartDate)<= @eDate 
  )
select * from cte
option (maxrecursion 0)

12、视图

很多人对视图View感到很沮丧，因为它看起来跟select语句没什么区别。在视图中我们同样可以使用select查询语句，但是视图对我们来说依然非常重要。

假设我们要联合查询4张表中的20几个字段，那么这个select查询语句会非常复杂。但是这样的语句我们在很多地方都需要用到，如果将它编写成视图，那么使用起来会方便很多。利用视图查询有以下几个优点：

  • 一定程度上提高查询速度
  • 可以对一些字段根据不同的权限进行屏蔽，因此提高了安全性
  • 对多表的连接查询会非常方便

下面是一个视图的代码例子：

CREATE 
VIEW viewname
AS
Select ColumNames from yourTable

Example : 
-- Here we create view for our Union ALL example
Create 
VIEW myUnionVIEW
AS
 SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,
    I.Price*D.Qty as TotalPrice
   FROM 
  Ordermasters as M Inner JOIN OrderDetails as D 
  ON M.Order_NO=D.Order_NO INNER JOIN ItemMasters as I 
  ON  D.Item_Code=I.Item_Code WHERE I.Price <=44 
Union ALL
 SELECT M.order_NO,M.Table_ID,D.Order_detail_no,I.Item_Name,D.Notes,D.Qty,I.Price,
    I.Price*D.Qty as TotalPrice
   FROM 
  Ordermasters as M Inner JOIN OrderDetails as D 
  ON M.Order_NO=D.Order_NO INNER JOIN ItemMasters as I 
  ON  D.Item_Code=I.Item_Code WHERE I.Price>44

-- View Select query
Select * from myUnionVIEW
-- We can also use the View to display with where condition and with selected fields 
Select order_Detail_NO,Table_ID,Item_Name,Price from myUnionVIEW where price >40

13、Pivot行转列

Pivot可以帮助你实现数据行转换成数据列，具体用法如下：

-- Simple Pivot Example 
SELECT * FROM ItemMasters 
PIVOT(SUM(Price)    
 FOR ITEM_NAME IN ([Chiken Burger], Coffee,Coke)) AS PVTTable

-- Pivot with detail example
SELECT *
FROM (
  SELECT 
    ITEM_NAME, 
    price as TotAmount 
  FROM ItemMasters

) as s
PIVOT
(
  SUM(TotAmount)
  FOR [ITEM_NAME] IN ([Chiken Burger], [Coffee],[Coke])
)AS MyPivot

14、存储过程

我经常看到有人提问如何在SQL Server中编写多条查询的SQL语句，然后将它们使用到C#程序中去。存储过程就可以完成这样的功能，存储过程可以将多个SQL查询聚集在一起，创建存储过程的基本结构是这样的：

CREATE PROCEDURE [ProcedureName]                       

AS                                
BEGIN
-- Select or Update or Insert query.
END

To execute SP we use
exec ProcedureName

创建一个没有参数的存储过程：

-- =============================================                                
-- Author   : Shanu                                
-- Create date : 2014-09-15                                
-- Description : To Display Pivot Data                            
-- Latest                                
-- Modifier  : Shanu                                
-- Modify date : 2014-09-15                                 
-- =============================================                                
-- exec USP_SelectPivot                 
-- =============================================                              
Create PROCEDURE [dbo].[USP_SelectPivot]   
AS                                
BEGIN                         
  DECLARE @MyColumns AS NVARCHAR(MAX),
  @SQLquery AS NVARCHAR(MAX)
-- here first we get all the ItemName which should be display in Columns we use this in our necxt pivot query
select @MyColumns = STUFF((SELECT ',' + QUOTENAME(Item_NAME) 
          FROM ItemMasters
          GROUP BY Item_NAME
          ORDER BY Item_NAME
      FOR XML PATH(''), TYPE
      ).value('.', 'NVARCHAR(MAX)') 
    ,1,1,'')
-- here we use the above all Item name to disoplay its price as column and row display
set @SQLquery = N'SELECT ' + @MyColumns + N' from 
       (
         SELECT 
    ITEM_NAME, 
    price as TotAmount 
  FROM ItemMasters
      ) x
      pivot 
      (
         SUM(TotAmount)
        for ITEM_NAME in (' + @MyColumns + N')
      ) p '

exec sp_executesql @SQLquery;     

  RETURN                       
  END

15、函数Function

之前我们介绍了MAX(),SUM(), GetDate()等最基本的SQL函数，现在我们来看看如何创建自定义SQL函数。创建函数的格式如下：

Create Function functionName
As
Begin
END

下面是一个简单的函数示例：                                                 

Alter FUNCTION [dbo].[ufnSelectitemMaster]()
RETURNS int 
AS 
-- Returns total Row count of Item Master.

BEGIN
 DECLARE @RowsCount AS int;

Select @RowsCount= count(*)+1 from ItemMasters
 RETURN @RowsCount;

END

-- to View Function we use select and fucntion Name
select [dbo].[ufnSelectitemMaster]()

下面的一个函数可以实现从给定的日期中得到当前月的最后一天：

ALTER FUNCTION [dbo].[ufn_LastDayOfMonth]
(
  @DATE NVARCHAR(10) 
)
RETURNS NVARCHAR(10)
AS
BEGIN
  RETURN CONVERT(NVARCHAR(10), DATEADD(D, -1, DATEADD(M, 1, CAST(SUBSTRING(@DATE,1,7) + '-01' AS DATETIME))), 120)
END
SELECT dbo.ufn_LastDayOfMonth('2014-09-01')AS LastDay

以上就是适合初学者学习的基础SQL查询语句，希望对大家学习SQL查询语句有所帮助。

鲜花
握手
雷人
路过
鸡蛋
该文章已有0人参与评论

请发表评论

全部评论

专题导读
More+
上一篇：
SQL数据库优化大总结之百万级数据库优化方案发布时间：2022-02-08
下一篇：
详解SQL中drop、delete和truncate的异同发布时间：2022-02-08
热门推荐
More+
热门话题
More+
阅读排行榜

扫描微信二维码

查看手机版网站

随时了解更新最新资讯

139-2527-9053

在线客服（服务时间 9:00～18:00）

在线QQ客服
地址：深圳市南山区西丽大学城创智工业园
电邮：jeky_zhao#qq.com
移动电话：139-2527-9053

Powered by 互联科技 X3.4© 2001-2213 极客世界.|Sitemap