![]() ![]() That would give the highest score for the even ids (where id % 2 equals 0), then the highest score the odd ids (where id % 2 equals 1). ORDER BY effective_date DESC) AS person_address_ordinalĪs for the question about what (a % 2) does, it's just a mathematical calculation for MOD(a, 2), so you could do the following. ROW_NUMBER() OVER (PARTITION BY person_id It is broadly equivalent to this following. Step 2 : filter to just the first row per person_id In addition to RDBMS capabilities, it provides indexes, views, stored procedures, triggers, atomicity features, and so on. It is famous for its Open-Source platform, which supports all RDBMS functionalities. ![]() Id | person_id | address_id | effective_date Image Source PostgreSQL is a high-performance, open-source relational database. Thus, giving the most recent address for each person. Then, for each person, on the first record is returned. This will order the results so that all the records for each person are contiguous, ordered from the most recent record to the oldest. The results are then filtered, so that for each of the distinct entities, only the first row is actually returned.Įxample (id, person_id, address_id, effective_date) SELECT DISTINCT ON (a) a, b, c FROM a_table ORDER BY a, b The first point is that whatever you put in the ON (), must come first in the the ORDER BY, for reasons that will hopefully shortly become clear. The DISTINCT ON expression(s) must match the leftmost ORDER BY expression(s). ![]() Note that the "first row" of each set is unpredictable unless ORDER BY is used to ensure that the desired row appears first. The snippet from the Official documentation for PostgreSQL is as follows.ĭISTINCT ON ( expression ) keeps only the first row of each set of rows where the given expressions evaluate to equal. While the previous answer appears correct, I don't feel that it is particularly clear. ![]()
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