How to delete duplicate entries?

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Some of these approaches seem a little complicated, and I generally do this as:

Given table table, want to unique it on (field1, field2) keeping the row with the max field3:

DELETE FROM table USING table alias 
  WHERE table.field1 = alias.field1 AND table.field2 = alias.field2 AND
    table.max_field < alias.max_field

For example, I have a table, user_accounts, and I want to add a unique constraint on email, but I have some duplicates. Say also that I want to keep the most recently created one (max id among duplicates).

DELETE FROM user_accounts USING user_accounts ua2
  WHERE user_accounts.email = ua2.email AND user_account.id < ua2.id;
  • Note – USING is not standard SQL, it is a PostgreSQL extension (but a very useful one), but the original question specifically mentions PostgreSQL.

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