SQL1:

select t.id,c.fullname as

from tbl_trans t

left join tbl_contacts c

on t.contactId = c.id

where t.name = "aaaa"

查询结果显示1个字段：{"id":"c6fc1fdb3f8f49298f652801df36a446"}

当交易表的联系人为空时，SQL1的结果只显示t.id，contactName为null被过滤掉

tbl_trans:

tbl_contacts:

SQL2:

select t.id,IFNULL(c.fullname,"") as contactName

from tbl_trans t

left join tbl_contacts c

on t.contactId = c.id

where t.name = "aaaa"

查询结果显示2个：

{"contactName":"","id":"c6fc1fdb3f8f49298f652801df36a446"}