MySQL 聯合查詢

合併查詢

子查詢

select field1,field2,…,fieldn

from tablename1 left
|right
[outer
]join tablename2
on condition;

select

*from student as a left
join class as b on a.class_id = b.id;
#左連線查詢所有學生對應的班級資訊
select
*from class as a left
join student as b on a.id = b.class_id;
#左連線查詢所有班級的學員資訊

select

*from student as a right
join class as b on a.class_id = b.id;
#右連線查詢所有班級對應的學員資訊
select
*from class as a right
join student as b on a.id = b.class_id;
#右連線查詢所有學員對應的班級資訊

select id, teacher as people from class union

allselect class_id, name as people from student;

使用比較運算子時，select 子句獲得的記錄數不能大於1條

select teacher from class where id=

(select class_id from student where name=
'小花'
);

乙個查詢語句的條件可能落在另乙個select語句的查詢結果中，這時可以使用in關鍵字

select teacher from class where id in

(select class_id from student where name=
'小花');
select teacher from class where id in
(select class_id from student where name like
'小%'
);

exists 返回的是真或假

select

*from class where id=
102and
exists
(select
*from student where class_id=
102)
;

關鍵字any表示滿足其中任一條件

select id, name, math+english+chinese total from grade where math+english+chinese >=

any(
select score from schoolarship)
;

關鍵字all表示滿足所有條件

select id, name, math+english+chinese total from grade where math+english+chinese >=

all(
select score from schoolarship)
;

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