仅在表2具有值时联接 [英] Join only when table 2 has a value

问题描述

是否可以组成一段SQL从两个表中检索数据，其中第二个表可能有值也可能没有值.

例如

Is it possible to compose a piece of SQL to retrive data from two tables where the second table may or may not have a value.

e.g.

 Select t1.name, t2.diplomatype
from studentmaster t1, resultmaster t2
where t1.result = t2.result
or t1.result is null

所需的输出量
姓名DiplomaType
---- -----------
汤姆·帕斯
玛丽·霍诺斯(Mary Honours)
杰克
丽兹通行证

...或者我是否需要使用存储的过程?

Desired Ouitput
Name DiplomaType
---- -----------
Tom Pass
Mary Honours
Jack
Liz Pass

... Or do I need to use a stored proocedure?

推荐答案

是的，当然，请使用MySQL外连接.

参见此处 [

Yes offcourse Use MySQL Outer JOIN.

See Here[^]

SELECT *
FROM
StudentMaster T1
    LEFT JOIN RESULTMASTER T2 ON
       T1.Result = T2.Result

您可以使用外部联接:

You may use an Outer Join:

Select t1.name, t2.diplomatype
from studentmaster t1 LEFT OUTER JOIN resultmaster t2
on  t1.code = t2.code
where 1=1

您必须将连接条件放在ON表达式中，然后完成.

You must put the join conditions in the ON expression, and it''s done.

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