关于mysql sum(if)的使用

刚才帮一同事处理一条很复杂的sql   用到了sum(if),具体的sql是

SELECT
sum(if(isinner=1,1,0 )) AS innerlinktotal,
  sum(if(isinner=0,1,0 )) AS outlinktotal,
  sum(if(isinner=1,sitmeta.down_size,0)) AS innerlinksize,
sum(if(isinner=0,sitmeta.down_size,0)) AS outlinksize,
  (sum(if(isinner=1,1,0 )) + sum(if(isinner=0,1,0 ))) AS total,
concat_ws(
ascription.operation,
`ascription`.`location`,
ascription.system
) AS belong
FROM
sitemeta_webkit_1490 AS sitmeta
LEFT JOIN domain_ascription_wwwiqiyicom AS ascription ON sitmeta.domain = ascription.domain
GROUP BY
ascription.location,
ascription.operation,
ascription.system;

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