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Does UK also serve as a FK?


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I have a junction table (child) linking two parent tables.

In the junction table I have a UK on 4 columns. 

The first column is the FK back to parent table-1.

The second column is the FK back to parent table-2.

In MySQL, can I just have a UK on the first 4 columns and have that satisfy the need for the UK plus the need to have indexes on columns that are FKs?

Or do I need to double up, and add a straight idx on column 1 and then on column 2 along with my 4-column UK - which is an index also.

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