SQL-SQL中查询结果带if函数返回数据集的方法

Question

select a.id, if(a.b_cnt > 0,(select b.id from b where b.a_id = 'test-1'), '') as b_id
from a
where a.id = 'test-1';
//两表无连接。仅b表中有a表id字段，a表中有b表含有a_id的统计数字b_cnt

Answer 1

select a.id, '' as b_id
from a
where a.id = 'test-1' and a.b_cnt <= 0
union all
select a.id, b.id as b_id
from a, b
where a.id = 'test-1' and b.a_id = 'test-1' and a.b_cnt > 0
//两表无连接。仅b表中有a表id字段，a表中有b表含有a_id的统计数字b_cnt

a, b表上 id都有索引的话, 效率应该没有问题.