一、 交集 join
1、inner join 其處理結果與等值/自然連線相同
mssql :
--mssql 中 inner join 連線需要 on connection條件 否則會報錯, mysql 不會,其結果等於cross join
select
*from major m inner
join
dbo.depnmorsubjectnm d
on m.depnoorsubjectno =
d.depnoorsubjectno
mysql:
--其結果等於 crose join
select
*from major m inner
join
department d ;
--等值連線
select m.*,d.*
from major m inner
join department d on m.dptid = d.dptid;
2、 left join /left outer join 左連線/左外連線
注：在sql 中, left join 為 left outer join 的縮寫
mssql:
select*from major m left
outer
join
dbo.depnmorsubjectnm d
on m.depnoorsubjectno =d.depnoorsubjectno
mysql:
select*from major m left
join `department` d on m.dptid = d.dptid;
3、 right join/right outer join 右連線/右外連線
mssql:
select*from major m right
outer
join
dbo.depnmorsubjectnm d
on m.depnoorsubjectno =d.depnoorsubjectno
mysql:
select*from major m right
outer
join `department` d on m.dptid = d.dptid;
4、full join 全連線
mssql:
--a select
*from major m full
outer
join
dbo.depnmorsubjectnm d
on m.depnoorsubjectno =
d.depnoorsubjectno --b
select
*from major m left
outer
join
dbo.depnmorsubjectnm d
on m.depnoorsubjectno =
d.depnoorsubjectno
union
select
*from major m right
outer
join
dbo.depnmorsubjectnm d
on m.depnoorsubjectno =
d.depnoorsubjectno
--c
select
*from major m left
outer
join
dbo.depnmorsubjectnm d
on m.depnoorsubjectno =
d.depnoorsubjectno
union
allselect
*from major m right
outer
join
dbo.depnmorsubjectnm d
on m.depnoorsubjectno =
d.depnoorsubjectno
where m.depnoorsubjectno is
null
mysql:
注：mysql 5.0版本還不支援 full join ,但可以採用 join + union方法實現,詳細可以參考:
--全連線 == 乙個左連線 + 乙個右連線 + union(去掉相同行) --a
select
*from major m left
outer
join `department` d on m.dptid =
d.dptid
union
select
*from major m right
outer
join `department` d on m.dptid =
d.dptid --b
select
*from major m left
outer
join `department` d on m.dptid =
d.dptid
union
allselect
*from major m right
outer
join `department` d on m.dptid =
d.dptid
where m.dptid is
null
--大力提倡使用b，尤其是在處理很大的記錄集時, union all 不會進行排序及消除相同的行(消除相同的行可能通過第二個join的條件進行實現),所以可以節省不少時間.
二、差集 (not in)
select*from major m where m.dptid not
in(select dptid from department )
差集中, mysql 與 mssql 語句可以直接採用 not in 來實現
三.、並集 union
mssql:
--union 並, 預設取消相同 行
select
*from
major m
union
select
*from
major tm
--union full 會有重覆記錄
select
*from
major m
union
allselect
*from major tm
mysql:
--union 缺省會取消重複選項
select
*from
major `major`
union
select
*from
major mj ;
--union all 不會取消重複選項
select
*from
major m
union
allselect
*from major tm
四、 笛卡爾積
mssql:
--笛卡爾積
select
*from major cross
join
dbo.depnmorsubjectnm
mysql:
--笛卡爾集
select
*from major m cross
join `department` d;
帶條件的笛卡爾積與等值連線功能相同
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