sql语句--关联查询

1，关联查询----sql中多个left join，为了保证返回数量和主表一样，要加个group by 主表id
2，if，ifnull，concat_ws等常见方法
1)concat_ws(’’,country, province, city) region 三字段按照’‘之间的内容合拼;
concat_ws(’’,CASE p.gameType1 WHEN 1 THEN ‘朗诵讲故事’ WHEN 2 THEN ‘朗诵情景演讲’ END,CASE p.gameType2 WHEN 3 THEN ‘主题写作’ END) as gameType;
2)if(gender=1,‘男’,‘女’) as gender;
3)ifnull(age,0) as age;
4)(CASE ageGroup WHEN 1 THEN ‘儿童A组’ WHEN 2 THEN ‘儿童B组’ WHEN 3 THEN ‘少年A组’ WHEN 4 THEN ‘少年B组’ END) as ageGroup;