1 交集 join
1) inner join 其處理結果與等值/自然連線相同
如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
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 all
select * from major m right outer join
dbo.depnmorsubjectnm d on m.depnoorsubjectno =d.depnoorsubjectno
where m.depnoorsubjectno is null
mysql:
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 all
select * 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.
2. 差集 (not in)
sql:
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
3. 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 all
select * 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 all
select * from major tm
4. 笛卡爾積
mssql:
-- 笛卡爾
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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