MySQL通过SQL语句进行递归查询

这里主要是针对于MySQL8.0以下版本，因为MySQL8.0版本出来了一个WITH RECURSIVE函数专门用来进行递归查询的

先看下表格数据，就是很普通的树结构数据，通过parentId关联上下级关系

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下面我们先根据上级节点id递归获取所有的下级节点数据，比如id为1

SELECT T2.id, T2.parent_id, T2.name
FROM (
    SELECT @r AS _id, 
           (SELECT @r := id FROM t_test WHERE parent_id = _id LIMIT 1) AS id
    FROM (SELECT @r := 1) vars,
         t_test h 
    WHERE @r IS NOT NULL
) T1 
JOIN t_test T2 ON T1._id = T2.parent_id
ORDER BY T2.id;

执行一下，可以看到可以获取到下级节点所有的数据

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这里有个问题就是没有拿到当前节点数据，我们改一下

SELECT 
    T2.id, 
    T2.parent_id, 
    T2.name
FROM 
    (SELECT @r AS _id, 
            @r := (SELECT id FROM t_test WHERE parent_id = _id LIMIT 1) AS id
     FROM 
            (SELECT @r := 1) vars,
            t_test
     WHERE 
            @r IS NOT NULL) T1 
JOIN 
    t_test T2 
ON 
    T1._id = T2.parent_id OR T1._id = T2.id
ORDER BY 
    T2.id;

运行一下
可以发现虽然拿到了当前节点，但是也出现了重复数据问题，目前还没想好要怎么改，所以只能在外层接收数据的时候处理，或者再套一层group by一下

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还有根据子节点查询所有上级节点的

SELECT T2.id, T2.parent_id, T2.name
FROM (SELECT @r AS _id, (SELECT @r := parent_id FROM t_test WHERE id = _id) AS parent_id
        FROM (SELECT @r := 5) vars,
        t_test h WHERE @r <> 0) T1 JOIN t_test T2 ON T1._id = T2.id

执行一下

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如果只想要顶层的

SELECT T2.id, T2.parent_id, T2.name
FROM (SELECT @r AS _id, (SELECT @r := parent_id FROM t_test WHERE id = _id) AS parent_id
        FROM (SELECT @r := 5) vars,
        t_test h WHERE @r <> 0) T1 JOIN t_test T2 ON T1._id = T2.id
ORDER BY T2.id LIMIT 1;