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To prevent duplicate table entries in a database I use a primary key. I just add the information and if it is a duplicate then the primary will be a duplicate and it will not add to the table.
Should I also do a SQL query (before trying to add to the database) to see if the entry exists? Or is this redundant since I already have the primary key set?
It is redundant to check for presence of a value if you already have a constraint to prevent duplicates.
But it would also be ineffective to check before you insert, because some other concurrent client might insert that value in the moment between your check and your insert. So even if you check first, you’d still need to handle duplicate key errors.