In MySQL Association query, the foreign key is null. What if the data cannot be found?

The foreign key is null Can you not associate the table , Change to normal query , Can it be realized ？

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Take the answer ：

The design concept of foreign key is to reasonably restrict the entry of data , Do not allow illegal data to be entered , When querying , There is no limit .

The data that the subject said cannot be found , Two tables should be directly executed join Caused by association , It is recommended to use left join Associate two tables , Common query （ Be sure to display the data ） The watch , Put it on the left side. , The table to be associated can be placed on the right ：

select * from table_1 a left join table_2 b on a.key = b.key 

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