Some time ago I wrote about ensuring that there is only one row with a specific value in a column.
Today a very similar problem: there is a table like this:
CREATE TABLE test ( id SERIAL PRIMARY KEY, name TEXT NOT NULL, value TEXT NOT NULL );
We need to be sure that there is only one row withing specific name with value=’a’. There can be more rows with this name, but only one can have value ‘a’.
For achieving that requirement I’ve created an index:
CREATE UNIQUE INDEX i_test ON test (name, value) WHERE value = 'a';
- This index is made only on the rows where value = ‘a’.
- For all those rows (and remember: there should be only one such row) this index contains only the pair of columns (name, value).
- This index is UNIQUE so there can only be unique values (in this case: only one row with the value true).
- Pair of columns is unique only if both arguments are unique.