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WITH AS短语，也叫做子查询部分（subquery factoring），是用来定义一个SQL片断，该SQL片断会被整个SQL语句所用到。
WITH t AS (
SELECT user_id,SUM(grade_num) AS grade_sum
FROM (SELECT user_id,grade_num * IF(type= 'add',1,-1) AS grade_num
FROM grade_info )AS t1
GROUP BY user_id )
SELECT id,name,grade_sum
FROM t
JOIN user u ON u.id = t.user_id
WHERE grade_sum = (SELECT MAX(grade_sum) FROM t)
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