新人求大神解释解释sql语句

[qq_22465835]

select t.*,( case when (select count(1) from t_xdt_usual_question_type t1 where  t1.PARENT_TAG=t.issue_TYPE )>0 then 'true' else 'false' end ) 
as isparent  from t_xdt_usual_question_type t where 1=1;

看不懂这个sql语句 请大神指教

[bw555]

select t.*,( case when (select count(1) from t_xdt_usual_question_type t1 where t1.PARENT_TAG=t.issue_TYPE )>0 then 'true' else 'false' end ) as isparent from t_xdt_usual_question_type t where 1=1; case when 条件1 then 结果1 else 结果2 end的用法 当满足条件的时候，返回结果1，不满足返回结果2

[wangwei]

select t.*, (case when (select count(1) from t_xdt_usual_question_type t1 where t1.PARENT_TAG = t.issue_TYPE) > 0 then 'true' else 'false' end) as isparent from t_xdt_usual_question_type t where 1 = 1; 查询表t_xdt_usual_question_type 所有字段以及isparent字段 isparent字段显示有下面查询确定 如果查询select count(1) from t_xdt_usual_question_type t1 where t1.PARENT_TAG = t.issue_TYPE) > 0 结果大于0 isparent字段显示为true 否则显示为false