[英]how to use CASE statement in MYSQL to select a row from results based on condition?
I am new to sql. I have a SELECT query that returns 2 rows which have a rid (1 in both rows) and a path column which has binary value 0 or 1 I want to use a CASE statement such that it will SELECT one row from results of first query based on condition s1.pid < s2.pid THEN (SELECT S2.rid WHERE r.path = 0) ELSE (SELECT S2.rid WHERE r.path=1 )
Below code returns two rows with rid 1 and path 0 and 1 how do i write a case statement?我是 sql 的新手。我有一个 SELECT 查询,它返回 2 行,其中有一个 rid(两行均为 1)和一个路径列,其二进制值为 0 或 1 我想使用 CASE 语句,这样它将 SELECT 一行从基于条件s1.pid < s2.pid THEN (SELECT S2.rid WHERE r.path = 0) ELSE (SELECT S2.rid WHERE r.path=1 )
的第一个查询的结果中,下面的代码返回两行 rid 1 和路径 0 和 1 我如何编写案例陈述? I dont know the correct syntax.我不知道正确的语法。
Schema: database schema架构:数据库架构
this is the result im getting from running 1st query now i want to select one of the rows based on condition where s1.pid< s2.pid then select row where path = 0 else otherwise how do i do that?这是我从运行第一个查询得到的结果,现在我想要 select 基于条件的行之一 where s1.pid< s2.pid 然后 select 行 where 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')
";
I've tried this but it is giving me error Unknown column 's2.pid' in 'field list'.我已经试过了,但它给了我错误 Unknown column 's2.pid' in 'field list'。 It seems that I cant access inner subquery alias in outer query or am I doing it wrong?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? what is the scope of s2? s2的scope是什么? Is there any work around for this?有什么解决方法吗?
$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
";
Edit 2: I've tried code below and it is returning p1.pid and p2.pid but giving syntax error in IF statement can anyone solve it?编辑 2:我试过下面的代码,它返回 p1.pid 和 p2.pid 但在 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'
";
You description is really confusing.你的描述真的很混乱。 Anyways, I have attempted to rewrite the query.无论如何,我试图重写查询。 Instead of using而不是使用
AND s2.rid IN (
SELECT s1.rid
I have used a self join with the same stop
table and alias it as s1
and used the join condition as s2.rid = s1.rid and s1.place = 'place1'
我使用了具有相同stop
表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'"
You can also simplify this further instead of using multiple IF or CASE statements.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 )
You can define it using r.path in (0,1)
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)
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