一道SQL小题

 

 

boss给妹子出了道小题，没事给她解了：

update test_123 as a
inner join (SELECT pid,COUNT(sons) count FROM test_123 GROUP BY pid ) as b on a.id = b.pid
set a.sons = b.count;

 

顺便还查了查update里插if 和 case的语法，有意思