12/4/2023 0 Comments Distinct on postgresql![]() When custom React Hooks do not rerender Components on their own – make them.This weblog contains public wisdom of our company Softwareschneiderei GmbH in Karlsruhe, Germany. We get the same result: name │ category │ value The ORDER BY clause determines which row will be the first in each group. How does it work? SELECT DISTINCT ON (category) *Īfter DISTINCT ON we specify one or more columns by which to group by in parentheses. If you then want to order the resulting records, make the DISTINCT ON a subselect and order its results: SELECT FROM ( SELECT DISTINCT ON (addressid) purchases.addressid, purchases. The easy wayīut I promised it can be easier with the DISTINCT ON clause. Your ORDER BY is used by DISTINCT ON for picking which row for each distinct addressid to produce. PostgreSQL has a really interesting and powerful construct called SELECT DISTINCT ON. It can be used with or without an ORDER BY clause. ) WHERE rownum = 1 name │ category │ value DISTINCT is a PostgreSQL specific keyword used to remove duplicate rows from the result set of a query. PostgreSQL DISTINCT ON with different ORDER BY SELECT FROM ( SELECT DISTINCT ON (col1) col1, col2, col3 FROM test ORDER BY col1, col3 DESC ) sub ORDER BY col3 DESC, col2 Assuming that col2 functionally depends on col1, so we can ignore it in DISTINCT ON and ORDER BY of the inner query. In this case we might use the ROW_NUMBER() function: SELECT But we really only want one row for each category. Unfortunately, there are multiple items in category X with the same highest value 52. Ive got the following SQLA code: ors ors.append(func.lower(pwEntries.uname). GROUP BY category category │ highest_valueĪnd then use this query as a sub-select: SELECT * FROM items I see a note in a change log that this is possible. Usually we’d start out with a query like this: SELECT Given the following table of items we want to query for each category the item with the highest value. ![]() To understand its usefulness, let’s look at an example and solve it in the classical way first. It can be used to query only the first row of each set of rows according to a grouping. However, PostgreSQL has another variant of it that not everyone knows, but which is very useful: the SELECT DISTINCT ON clause. So I would correct it to this: SELECT t1.col4, t1.col5įROM tablename t1 INNER JOIN (SELECT col1, col2, col3, MIN(col4) as m_col4īut this is starting to get more complicated.Īs a general rule, there's not an exact way to convert a Postgresql query to a MySQL query, but there are a lot of workarounds, the resulting query might be as simple as the original one or it might become very complicated, but it depends on the query itself.Anyone who uses SQL databases knows the DISTINCT modifier for SELECT queries to get result sets without duplicates. The idea is good, but it's wrong! MySQL is free to choose any value for col4 and col5, and we don't know which are the first values encountered, it depends on the optimizer. The idea here is to apply an ORDER BY to a subquery so that when MySQL groups by col1, col2, col3 it will keep the first encountered value for col4 and col5. Inside a table, a column often contains many duplicate values and sometimes. So this Postgresql query: SELECT DISTINCT ON (col1, col2, col3) col4, col5Ĭan be considered equivalent to this MySQL query: SELECT col4, col5īoth Postgresql and MySQL will return the "First row" for each (col1, col2, col3), and in both cases the row returned is unpredictable because we didn't specify and order by clause.Ī lot of people would be very tempted to convert this Postgresql query with an ORDER BY: SELECT DISTINCT ON (col1, col2, col3) col4, col5 postgres: Upgraded RDS Postgres from 9.4 - 9. The SELECT DISTINCT statement is used to return only distinct (different) values. ![]() Whenever we select nonaggregated columns the server is free to choose any value from each group from that column, so the resulting values will be indetermined. MySQL extends the use of GROUP BY so that we can select nonaggregated columns not named in the GROUP BY clause. (the DISTINCT on expressions must match the leftmost ORDER BY expressions, but the ORDER BY can contain additional expressions). Please notice that the "first row" of each set is unpredictable, our fist row might be (888, 999) as well unless we specify an ORDER BY: SELECT DISTINCT ON (col1, col2, col3) col4, col5 The resulting rows will then be: col4 | col5 Our query will keep just one row for (1,2,3) and one row for (3,3,3). So if your table is like this: col1 | col2 | col3 | col4 | col5 In Postgresql, the following query will eliminate all rows where the expressions (col1, col2, col3) match, and it will only keep the "first col4, col5 row" for each set of matched rows: SELECT DISTINCT ON (col1, col2, col3) col4, col5 There's not an exact equivalent to convert a Postgresql query that makes use of SELECT DISTINCT ON to MySQL.
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