如何使用 MYSQL 中的 CASE 语句根据条件从结果中 select 一行？

Question

我是 sql 的新手。我有一个 SELECT 查询 returns 2 行有一个 rid（两行中有 1 个）和一个具有二进制值 0 或 1 的路径列我想使用 CASE 语句，这样它将 SELECT 基于条件 s1.pid < s2.pid THEN (SELECT S2.rid WHERE r.path = 0) ELSE (SELECT S2.rid WHERE r.path=1 ) 的第一个查询结果的一行下面的代码 returns 两行带有 rid 1 和路径 0 和 1 我如何编写 case 语句？我不知道正确的语法。

架构：database schema

stopno rid busno pid 总线键路径
4 1 1111 4 键 1 0
4 1 2222 4 键 2 1

这是我从运行第一次查询得到的结果，现在我想 select 根据条件 s1.pid< s2.pid 然后 select 行，其中 path = 0 否则我该怎么做？

$sql="SELECT s2.stopno, s2.rid, busno, s2.pid, buskey, `path`
FROM `stop` s2
INNER JOIN `route` r ON s2.rid = r.rid
INNER JOIN bus b ON r.bid = b.bid
INNER JOIN place p ON s2.pid = p.pid
WHERE p.place = 'place2'
AND s2.rid IN (SELECT  s1.rid
                 FROM `stop` s1
                 INNER JOIN `route` r ON s1.rid = r.rid
                 INNER JOIN bus b ON r.bid = b.bid
                 INNER JOIN place p ON s1.pid = p.pid
                 AND p.place = 'place1')
";

我试过了，但它给我错误 'field list' 中的未知列 's2.pid'。似乎我无法在外部查询中访问内部子查询别名，或者我做错了吗？ s2的范围是什么？有什么解决办法吗？

$sql=" SELECT (CASE 
WHEN s2.pid > s1.pid
THEN (SELECT s3.rid 
     FROM `stop` s3
    INNER JOIN `route` r ON s3.rid = r.rid
    INNER JOIN bus b ON r.bid = b.bid
    INNER JOIN place p ON s3.pid = p.pid
    WHERE s3.rid IN 
          (SELECT s2.rid
          FROM `stop` s2
          INNER JOIN `route` r ON s2.rid = r.rid
          INNER JOIN bus b ON r.bid = b.bid
          INNER JOIN place p ON s2.pid = p.pid
          WHERE p.place = 'place2'
          AND s2.rid IN 
                (SELECT  s1.rid
                FROM `stop` s1
                INNER JOIN `route` r ON s1.rid = r.rid
                INNER JOIN bus b ON r.bid = b.bid
                INNER JOIN place p ON s1.pid = p.pid
                AND p.place = 'place1'))
                AND r.path = '0')
ELSE (SELECT s3.rid 
      FROM `stop` s3
      INNER JOIN `route` r ON s3.rid = r.rid
      INNER JOIN bus b ON r.bid = b.bid
      INNER JOIN place p ON s3.pid = p.pid
      WHERE  s3.rid IN
              (SELECT  s2.rid
              FROM `stop` s2
              INNER JOIN `route` r ON s2.rid = r.rid
              INNER JOIN bus b ON r.bid = b.bid
              INNER JOIN place p ON s2.pid = p.pid
              WHERE p.place = 'place2'
              AND s2.rid IN 
                    (SELECT  s1.rid
                    FROM `stop` s1
                    INNER JOIN `route` r ON s1.rid = r.rid
                    INNER JOIN bus b ON r.bid = b.bid
                    INNER JOIN place p ON s1.pid = p.pid
                    AND p.place = 'place1'))
                    AND r.path = '1')
                    END)AS pathfinder
FROM `stop` s
INNER JOIN `route` r ON s.rid = r.rid
INNER JOIN bus b ON r.bid = b.bid
INNER JOIN place p ON s.pid = p.pid
";

编辑 2：我试过下面的代码，它返回 p1.pid 和 p2.pid 但是在 IF 语句中给出语法错误任何人都可以解决它吗？

$sql="SELECT s2.stopno, s2.rid, b.busno, p1.pid as id1, p2.pid as id2, b.buskey, r.path, p1.place as place1, p2.place as place2,
   IF(p1.pid < p2.pid, IF(r.path=0, s2.*, null), IF(r.path=1, s2.*,null))
       
FROM `stop` s2

JOIN `route` r
ON s2.rid = r.rid
JOIN bus b 
ON r.bid = b.bid
JOIN place p1
ON s2.rid = p1.pid
JOIN place p2
ON s2.pid = p2.pid
WHERE 
p1.place = 'place1'
AND p2.place = 'place 2'

";

Answer 1

你的描述真是令人费解。无论如何，我试图重写查询。而不是使用

AND s2.rid IN (
                    SELECT  s1.rid

我使用了具有相同 stop table 的自连接并将其别名为 s1 并将连接条件用作 s2.rid = s1.rid and s1.place = 'place1'

$sql="SELECT s2.stopno, s2.rid, b.busno, s2.pid, b.buskey, r.path,
        IF(s1.pid < s2.pid, IF(r.path=0, s2.rid, null), IF(r.path=1, s2.rid,null))
    FROM `stop` s2, bus b, `route` r, place p, `stop` s1
    WHERE s2.rid = r.rid
    AND r.bid = b.bid
    AND s2.pid = p.pid
    AND s2.rid = s1.rid and s1.place = 'place1'
    AND p.place = 'place2'"

您还可以进一步简化它，而不是使用多个 IF 或 CASE 语句。

s1.pid < s2.pid THEN (SELECT S2.rid WHERE r.path = 0) ELSE (SELECT S2.rid WHERE r.path=1 )

您可以使用 r.path in (0,1)

来定义它

IF(s1.pid < s2.pid), IF(r.path in (0,1),s2.rid, NULL), NULL)

如何使用 MYSQL 中的 CASE 语句根据条件从结果中 select 一行？

how to use CASE statement in MYSQL to select a row from results based on condition?

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