SQL语句问题?

Question

select sum(case child_id when '' then 0 else 1 end) growth_photo_count,
          -1 as fc  from  t_growth_photo tgp 
     这个SQL如何理解?? 尤其是 sum(case child_id when '' then 0 else 1 end) growth_photo_count
          

Answer 1

如同if else

child_id为空按0统计，否则按1统计 ，统计的字段用growth_photo_count表示