select實用知識點

建表

create table waw_school
(
 id int identity(1,1) primary key,
 name nvarchar(60),
 age tinyint,
 phone bigint,
 hobby nvarchar(512),
 note nvarchar(1024),
 utime datetime default getdate()
);

create table waw_class
(
 id int identity(1,1) primary key ,
 school_id int,
 name nvarchar(60),
 note nvarchar(1024),
 utime datetime default getdate()
)

----------------------------------------------------------------

《一》distinct在查詢時候用到，用於對查詢結果排重
select distinct school_id from waw_class
等同於
select school_id from waw_class
group by school_id

《二》子查詢：在查詢中，子查詢結果集用於外圍查詢的限制條件
select * from waw_class a
where a.school_id in (select id from waw_school where name = 'zsq')
或
select * from waw_class a
where exists (select * from waw_school b where a.school_id = b.id and b.id = 1)
或
select * from waw_class a
where not exists (select * from waw_school b where a.school_id = b.id and b.id = 2)

《三》基於列的邏輯表達式
select a.id ,
a.school_id ,
case when a.school_id = 1 then 'zsq' when a.school_id = 2 then 'zxn' end as school,
a.name,a.note,a.utime
from waw_class a

《四》關於分組（group by）對於須要根據不一樣緯度統計時，只用group by就會很麻煩，例如：

下邊我結合我在實際開發中的例子，提出新的解決方案：

--a,b,c      abc  ab  a

SELECT CAST(a.採集時間 AS DATE) AS 發佈時間,a.正負面,a.信息類型,COUNT(1) FROM 監測執行表 a WITH(NOLOCK)

WHERE a.事件ID = 2014095560 AND a.採集時間 > '2017-05-10'

GROUP BY ROLLUP (CAST(a.採集時間 AS DATE),a.正負面,a.信息類型)

--a,b,c      abc  ab  ac  bc  a  b  c

SELECT CAST(a.採集時間 AS DATE) AS 發佈時間,ISNULL(a.正負面,'中性') AS 正負面,ISNULL(a.信息類型,4) 信息類型,COUNT(1) FROM 監測執行表 a WITH(NOLOCK)

WHERE a.事件ID = 2014095560 AND a.採集時間 > '2017-05-10'

GROUP BY CUBE (CAST(a.採集時間 AS DATE),ISNULL(a.正負面,'中性'),ISNULL(a.信息類型,4) )

--a,b,c   a   bc 

SELECT CAST(a.採集時間 AS DATE) AS 發佈時間,ISNULL(a.正負面,'中性') AS 正負面,ISNULL(a.信息類型,4) 信息類型,COUNT(1) FROM 監測執行表 a WITH(NOLOCK)

WHERE a.事件ID = 2014095560 AND a.採集時間 > '2017-05-10'

GROUP BY GROUPING SETS  (CAST(a.採集時間 AS DATE),(ISNULL(a.正負面,'中性'),ISNULL(a.信息類型,4) ) )

注意： 使用GROUPING SETS僅僅是一個能夠少寫些代碼的語法糖.但實際狀況是，GROUPING SETS在遇到多個條件時，聚合是一次性從數據庫中取出全部須要操做的數據，在內存中對數據庫進行聚合操做並生成結果。而UNION ALL是屢次掃描表，將返回的結果進行UNION操做