SQL:连接表也显示空值

Question

我有这样的表:

表格1:

id | item_name | entered_by | modify_by
1  | banana    |     2      |    1
2  | apple     |     4      |    3
3  | orance    |     1      |    1
4  | pineapple |     5      |    3
5  | grape     |     6      |    1

表2:

id | username 
1  | admin
2  | jack
3  | danny
4  | dummy
5  | john
6  | peter

查询可以正常工作,以选择enter_by或modify_by是否有值:

SELECT t1.id, t1.item_name,
  t2enteredBy.username enteredBy,
  t2modifyBy.username modifyBy
FROM table1 t1
JOIN table2 t2enteredBy ON t1.entered_by = t2enteredBy.id
JOIN table2 t2modifyBy ON t1.modify_by = t2modifyBy.id

问题:如果modifiy_by或entered_by字段中的一个具有空值,则该行现在显示出来,如果它具有空值而不是完全隐藏该行,我需要它将其显示为" - ".

SQLFIDDLE在这里

Answer 1

试试这个:

SELECT t1.id, t1.item_name,
  COALESCE(t2enteredBy.username, '-') enteredBy,
  COALESCE(t2modifyBy.username, '-') modifyBy
FROM table1 t1
LEFT JOIN table2 t2enteredBy ON t1.entered_by = t2enteredBy.id
LEFT JOIN table2 t2modifyBy ON t1.modify_by = t2modifyBy.id

小提琴这里.

您需要left join使用null值返回这些行.然后,coalesce如果它们是,则确保它们被给定的字符串替换null.