正确的方法:
SELECt user, IF(SUM(group = 'A'), TRUE, FALSE) AS IN_A, IF(SUM(group = 'B'), TRUE, FALSE) AS IN_B, IF(SUM(group = 'C'), TRUE, FALSE) AS IN_CFROM usersGROUP BY user

正确的方法:
SELECt user, IF(SUM(group = 'A'), TRUE, FALSE) AS IN_A, IF(SUM(group = 'B'), TRUE, FALSE) AS IN_B, IF(SUM(group = 'C'), TRUE, FALSE) AS IN_CFROM usersGROUP BY user