GORM distinct() + count() 的问题

当我们想要查询 “不同记录的条数” 时，我们可能会这样写：

int totalCampaignUser = CampaignParticipation.where {
    eq "campaign.id", campaignToJoin.id
}.distinct("user").count()?.intValue() ?: 0

但这样会生成一个错误的 SQL

select distinct this_.user_id as y0_, count(*) as y1_ from campaign_participation this_ where this_.campaign_id=? limit ?

原因是用了聚合函数count(*)，但 select 子句中的字段"user_id"没有出现在 group by 中。

报错信息：

2020-04-30 13:28:00.351 DEBUG --- [    Test worker] org.hibernate.SQL                        : select distinct this_.user_id as y0_, count(*) as y1_ from campaign_participation this_ where this_.campaign_id=? limit ?
2020-04-30 13:28:00.351 TRACE --- [    Test worker] o.h.type.descriptor.sql.BasicBinder      : binding parameter [1] as [BIGINT] - [188]
2020-04-30 13:28:00.446 ERROR --- [    Test worker] o.h.engine.jdbc.spi.SqlExceptionHelper   : In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'chess_test.this_.user_id'; this is incompatible with sql_mode=only_full_group_by

解决办法：

可以用 countDistinct()，例如这样：

def c = Account.createCriteria()
def numberOfBranches = c.get {
    projections {
        countDistinct('branch')
    }
}

具体项目中，试验成功的代码：

Number totalCampaignUser = CampaignParticipation.where {
    eq "campaign.id", campaignToJoin.id
    projections {
        countDistinct "user"
    }
}.get() as Number ?: 0