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                合規國際互聯網加速 OSASE為企業客戶提供高速穩定SD-WAN國際加速解決方案。 廣告
                1.普通聯表的語句 select a.*,b.* from A a ,B b where a.id=b.id; 2.內連接查詢 select a.* ,b.* from A a inner join B b on a.id=b.id; A表和B表值相等的全匹配。 3.左連接查詢 select a.* ,b.* from A a left join B b on a.id=b.id; 以左表為基準表,查詢出來的結果等值的數據和左表不能匹配的右表的數據為null顯示。 4.右連接查詢 select a.* ,b.* from A a right join B b on a.id=b.id; 以右表為基準表,查詢出來的結果等值的數據和右表不能匹配的左表的數據為null顯示。 5.全連接 select a.* ,b.* from A a full join B b on a.id=b.id; 等值的結果和左表不匹配的結果和右表不匹配的結果都顯示。 舉例: 1.每個班級的學生的男,女數量。 SELECT COUNT(ssex),s.ssex,a.cname FROM t_classes a INNER JOIN t_stus s ON a.cid=s.scid GROUP BY a.cname,s.ssex 2. 把姓名重復的顯示出來 SELECT u.uname ,COUNT(u.uname) FROM t_userinfo u GROUP BY u.uname HAVING COUNT(u.uname)>1 3. 把姓名重復的過濾 SELECT DISTINCT(uname) FROM t_userinfo
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