SELECT * FROM classes;
id name
1 一班
2 二班
3 三班
4 四班
SELECT * FROM students;
id class_id name gender
1 1 小明 M
2 1 小紅 F
3 1 小軍 M
4 1 小米 F
5 2 小白 F
6 2 小兵 M
7 2 小林 M
8 3 小新 F
9 3 小王 M
10 3 小麗 F
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找出每個班級的名稱及其對應的女同學數量
#正確
SELECT c.name, count(s.name) as num
FROM classes c left join students s
on s.class_id = c.id
and s.gender = 'F'
group by c.name
或者
#錯誤
SELECT c.name, count(s.name) as num
FROM classes c left join students s
on s.class_id = c.id
where s.gender = 'F'
group by c.name
2.找出一班的同學總數
#正確
SELECT c.name, count(s.name) as num
FROM classes c left join students s
on s.class_id = c.id
where c.name = '一班'
group by c.name
或者
#錯誤
SELECT c.name, count(s.name) as num
FROM classes c left join students s
on s.class_id = c.id
and c.name = '一班'
group by c.name
總結：在left join語句中，左表過濾必須放where條件中，右表過濾必須放on條件中