多个 join 串联的问题! 奇怪
a (id1,id2)
b (id1)
c (id1,id2)
SELECT COUNT(*) FROM a
得到结果:1901
SELECT COUNT(*) FROM a LEFT OUTER JOIN b ON a.ID1 = b.ID1
得到结果:1901
SELECT COUNT(*) FROM a LEFT OUTER JOIN b ON a.ID1 = b.ID1 LEFT OUTER JOIN c ON a.ID2 = c.ID2
得到结果:2166
SELECT COUNT(*) FROM a LEFT OUTER JOIN b ON a.ID1 = b.ID1 LEFT OUTER JOIN c ON a.ID2 = c.ID2 and a.ID1=c.ID1
得到结果:1901
请问,为什么我连续用2个 LEFT JOIN,得到的结果反而比用1个LEFT JOIN 的结果多呢?
连续的LEFT JOIN,不是应该以最左边的表的条数为准的吗???