mysql查询left join

MySQL中的左连接（left join）是一种连接操作，它返回左表中的所有行，以及满足连接条件的右表中的匹配行。

对于需求1，正确的SQL写法是：
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
这个查询会返回每个班级的名称和女生人数。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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