SQL基礎知識總結（一）

1.union 和union all 操做符

 1）union內部的select語句必須擁有相同的列，列也必須有類似的數字類型。同時，每條select語句中列的順序相同。

 union語法（結果集無重複）

select x from table1

union 

select y from table2

 union all語法（結果集有重複）

select x from table1

union all

select y from table2

 

 2.CTE(Common Table Expression)

  公共表表達式，能夠認爲是在單個 SELECT、INSERT、UPDATE、DELETE 或 CREATE ⅥEW 語句的執行範圍內定義的臨時結果集

   CTE可用於：

  1).建立遞歸查詢

  2).在同一語句中屢次生成的表

 把test表中salary最大的id記錄保存在test_CTE中，再調用

with test_CTE(id,salary)
　　as
　　（
　　　　select id,max(salary)
　　　　from test
　　　　group by id
　　 ）
　　select * from test_cte

 查詢ItemId=2及子節點，也就是管理費用和其下屬全部節點的信息

Declare @i int
select @i=2;
with Co_ItemNameSet_CTE(ItemId,ParentItemId,ItemName,[Level])
AS
(
  select ItemId, ParentItemId,ItemName,1 AS [Level] 
  from Co_ItemNameSet
  where  ItemId=@i
  union all
  select c.ItemId,c.ParentItemId,c.ItemName,[Level]+1
  from Co_ItemNameSet c inner join Co_ItemNameSet_CTE ct
  on c.ParentItemId=ct.ItemId
)
select * from Co_ItemNameSet_CTE

結果：

3.row_number() over()

--不用partition by
select name,Gender,FenShu, row_number()over(order by FenShu desc) as num  from dbo.PeopleInfo

--使用partition by
select name,Gender,FenShu, row_number()over(partition by Gender order by FenShu desc) as num  from dbo.PeopleInfo
;
--查找出不一樣性別中分數最高的學生
with temp
as
(
  select name,Gender,FenShu, row_number()over(partition by Gender order by FenShu desc) as num  from dbo.PeopleInfo

)
select * from temp where num=1

4.Join

 從兩個或更多表中獲取結果，就要執行Join

  Inner Join

 

select P.LastName,P.FirstName, O.OrderNo from dbo.Persons as P

inner join dbo.mOrder as O

on P.Id= O.Id_P

order by P.LastName

 結果：
 

 left Join

select c.customerid as 消費者,COUNT(O.[orderid]) as  訂單數
  
from [SQLDemo].[dbo].[Customers] as C left join [SQLDemo].[dbo].

[Orders] as O 
  
on C.customerid=O.customerid  where  C.city='Madrid'
  
  
group by C.customerid  having COUNT(O.orderid)<3
  
order by 訂單數

5.SQL通配符

/****** SQL 通配符 ******/
select * from Persons where City like '[ALN]%'