SELECT a.`id`,a.`name`,a.`sex`,b.`indentity` ,c.`Class_id`FROM s1 a LEFT JOIN s2 b ON a.`id`=b.`id` LEFT JOIN s3 c ON a.`id`=c.`id` UNION SELECT e.`id`,e.`name`,f.`sex`,g.`indentity`,e.`Class_id` FROM s3 e LEFT JOIN s1 f ON e.`id`=f.`id` LEFT JOIN s2 g ON e.`id`=g.`id` ORDER BY id 方法比较笨,好的也想不出来,可以参考一下。😂
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