Mysql Unique constraints across 2 columns

Question

I just started using sqlalchemy so my question below might not make sense but bear with me. I have two columns in my table colA and colB. I want elements of colA and colB to be unique across both the columns. For example if,

     ColA  ColB
row1 1     2 
row2 3     2 X(2 is already present in row1 ColB, therefore is a duplicate)
row3 4     1 X(1 is already present in row1 ColA, therefore is a duplicate)

then row2 and row3 in the example above must not be allowed. How do I do this in sqlalchemy? I have already researched about this and looked at other similar question but haven't found a suitable answer yet.

Answer 1

Use your DBMS to create a two-column unique index.

ALTER TABLE mytable ADD UNIQUE INDEX myindex (colA, colB);

If I understand your question correctly, this will constrain your table the way you want.