使用UNION ALL保留结果

时间:2013-06-08 00:54:25

标签: mysql sql

我有一个使用UNION ALL关键字创建的查询。问题是,当其中一个SELECT返回一个空集时,它将被丢弃。我需要它至少返回NULL或空行。

如果你为每个SELECT都有一个唯一的索引,我会在某处读到它,它会阻止丢弃,但它无效。

使用循环创建查询,因此日期索引(即“2013-Jun”)会发生变化,只显示一个选择。

SELECT `products`.`id`, `products`.`ugid`, CONCAT(`products`.`fsg_pre`, `products`.`fsg_suff`) AS `fsg`, `year`, 'Mar' AS `month`, '2012-Mar' AS `index`, `Mar` AS `price`
FROM `prices`
JOIN `products`
ON (`products`.`uuid` = `prices`.`uuid`)
WHERE TRUE
AND `products`.`ugid` = '52777'
AND `products`.`fsg_pre` = 'SU'
AND `products`.`fsg_suff` = '3'
AND `year` = 2012

UNION ALL
[..........]

1 个答案:

答案 0 :(得分:0)

为什么不按照IMSoP的建议使用LEFT JOIN,如:

SELECT `products`.`id`, `products`.`ugid`, CONCAT(`products`.`fsg_pre`,
`products`.`fsg_suff`) AS `fsg`, `year`, 'Mar' AS `month`, '2012-Mar' AS `index`, 
`Mar` AS `price`
 FROM `prices`
 LEFT JOIN `products`
 ON `products`.`uuid` = `prices`.`uuid`
 WHERE `products`.`ugid` = '52777'
 AND `products`.`fsg_pre` = 'SU'
 AND `products`.`fsg_suff` = '3'
 AND `year` = 2012

或FULL JOIN总是空行或不行。

 SELECT `products`.`id`, `products`.`ugid`, CONCAT(`products`.`fsg_pre`,
`products`.`fsg_suff`) AS `fsg`, `year`, 'Mar' AS `month`, '2012-Mar' AS `index`, 
`Mar` AS `price`
 FROM `prices`
 FULL JOIN `products`
 ON `products`.`uuid` = `prices`.`uuid`
 WHERE `products`.`ugid` = '52777'
 AND `products`.`fsg_pre` = 'SU'
 AND `products`.`fsg_suff` = '3'
 AND `year` = 2012

请参阅链接here,直观地了解JOIN的工作原理。