最近做項(xiàng)目遇到一個(gè)問題，外鍵關(guān)聯(lián)查詢，返回長度為1，但是值為 All elements are null沒數(shù)據(jù)。
有兩張表，分別為a表有數(shù)據(jù)，b表無數(shù)據(jù)。
select b.* from a as a left join b as b on b.a_id = a.id 發(fā)生異常
修正sql select b.* from a as a left join b as b on b.a_id = a.id where b.id is not null就可以避免這種情況。
select a.* from a as a left join b as b on b.a_id = a.id 無異常，
