在MySQL选择查询中嵌套if / else

Question

I am having a bit of trouble getting my syntax right for a nested mysql if statement. This is my current query which is fine. But what I need is the parent category. Heres where it confuses me.

Basically I need to add the parentid bit to my query (written in pseudo-code)

      SELECT  p.*,i.`image`,

            ( IF(`limitc1`.`parentid` IN (135,136), `limitc1`.parentid)
              ELSEIF(`limitc2`.`parentid` IN (135,136), `limitc2`.parentid)
              ELSEIF(`limitc3`.`parentid` IN (135,136), `limitc3`.parentid)
              ELSEIF(`limitc4`.`parentid` IN (135,136), `limitc4`.parentid)
            ) as `parentid`
            FROM `product` p
             JOIN `productcategory` limitpc USING(productid)
                LEFT JOIN `category` limitc1 ON (limitpc.`categoryid` = limitc1.`categoryid`)
                LEFT JOIN `category` limitc2 ON (limitc1.`parentid` = limitc2.`categoryid`)
                LEFT JOIN `category` limitc3 ON (limitc2.`parentid` = limitc3.`categoryid`)
                LEFT JOIN `category` limitc4 ON (limitc3.`parentid` = limitc4.`categoryid`)
            LEFT JOIN `productlink` l ON (p.`productid` = l.`targetid` OR p.`productid` = l.`sourceid`)
            JOIN `productimage` i ON p.`productid` = i.`productid` AND i.`primary` = 1
            WHERE (l.`sourceid` = 5471 OR l.`targetid` = 5471)
            AND p.`visible` = 1
            AND p.`websitevisible` = 1
            AND p.`productid` != 5471
            GROUP BY p.`productid`
            LIMIT 8

Any ideas would be greatly appreciated! :-)

Answer 1

You want to use the case statement:

 SELECT  p.*,i.`image`,
         (case when `limitc1`.`parentid` IN (135,136) then `limitc1`.parentid
               when `limitc2`.`parentid` IN (135,136) then `limitc2`.parentid
               when `limitc3`.`parentid` IN (135,136) then `limitc3`.parentid
               when `limitc4`.`parentid` IN (135,136) then `limitc4`.parentid
         end) as `parentid`
  . . .

Not only does this do what you want to do, but it is also standard SQL. The if statement is specific to MySQL.