在带有PDO的PHP中，用变量联合选择。 怎么做对？

Question

I am trying to run a query that returns a random row, and 2 rows surrounding that row (previous and next).

I have saved the random result value in a variable so it can be used by all the following queries.

My query is as follows:

SET @rand := (SELECT CEIL(RAND() * MAX(id)) FROM data); 
(SELECT * FROM `data` where `id` < @rand AND safe = 1 ORDER BY ID DESC LIMIT 1) UNION 
(SELECT * FROM `data` where `id` = @rand AND safe = 1 ORDER BY ID DESC LIMIT 1) UNION
(SELECT * FROM `data` where `id` > @rand AND safe = 1 ORDER BY ID ASC LIMIT 1);

the @rand does contain a valid random ID.

It turns out to have been a display issue on the phpmyadmin interface I had been using from a university pc.

The query executes correctly and in the mysql command line interface the rows are shown correctly. However in PHP/pdo it now returns

PDOStatement::fetchAll(): SQLSTATE[HY000]: General error

whereas the same code without @rand in the query and using a static id for testing purposes, it returns the 3 rows as expected.

Answer 1

You have set that query such that it will return at max 3 rows as you have used LIMIT 1 for each 3 queries.

The other thing, despite this, you have also applied filtering to these query using

where id > @rand

So, how can you expect more than 3 rows ?

May be to get previous & next row, you can use this logic as,

SELECT * FROM `data` where `id` < @rand ORDER BY ID DESC LIMIT 1
UNION
SELECT * FROM `data` where `id` = @rand LIMIT 1
UNION
SELECT * FROM `data` where `id` > @rand ORDER BY ID ASC LIMIT 1

Will work little bit better.

Answer 2

You can get your result by this way

SELECT * 
FROM `data` 
where `id` between @rand-1 and @rand+

Ok. Then you can try this

SET @rand := (SELECT CEIL(RAND() * MAX(id)) FROM data);

 SELECT top 1 * FROM `data` where `id` < @rand AND safe = 1 ORDER BY ID DESC
 UNION 
 SELECT top 1 * FROM `data` where `id` = @rand AND safe = 1 
 UNION 
 SELECT top 1 * FROM `data` where `id` > @rand AND safe = 1 ORDER BY ID ASC

Answer 3

I suspect that PDO is having trouble running two SQL statements. I would create a stored procedure to run the statements and use PDO to call the stored proc:

CREATE PROCEDURE spGetThreeRandomRows()
BEGIN
    @rand := (SELECT CEIL(RAND() * MAX(id)) FROM data); 
    (SELECT * FROM `data` where `id` < @rand AND safe = 1 ORDER BY ID DESC LIMIT 1) UNION 
    (SELECT * FROM `data` where `id` = @rand AND safe = 1 ORDER BY ID DESC LIMIT 1) UNION
    (SELECT * FROM `data` where `id` > @rand AND safe = 1 ORDER BY ID ASC LIMIT 1);
END; 

Then call it like this:

$stmt = $dbh->prepare("CALL spGetThreeRandomRows()");
$stmt->execute();