To illustrate this usage, let’s retrieve a list of the average test scores for each department. Remember how this wasn't possible? The PostgreSQL GROUP BY clause is used in collaboration with the SELECT statement to group together those rows in a table that have identical data. The same technique can be used to allow a query to benefit from an index which has a leading column with few distinct values (and which would not naturally be specified in the query), and a 2nd column which is highly specific and is used by the query. Syntax #2. This is done to eliminate redundancy in the output and/or compute aggregates that apply to these groups. The DISTINCT ON clause will only return the first row based on the DISTINCT ON(column) and ORDER BY clause provided in the query. -----Original Message----- From: John Liu Sent: Wednesday, March 31, 2004 11:50 AM To: 'pgsql-general@postgresql.org' Subject: select distinct w/order by I know this is an old topic, but it's not easy to find a way around it, so when we migrate SQL from other database to PostgreSQL… Using PostgreSQL SELECT DISTINCT clause. » PostgreSQL problem: SELECT DISTINCT, ORDER BY: Sorry for messing with the title of the issue. Well, with DISTINCT ON, we just want PostgreSQL to return a single row for each distinct group defined by the ON clause. The EXCEPT operator computes the set of rows that are in the result of the left SELECT statement but not in the result of the right one. The GROUP BY clause follows the WHERE clause in a SELECT statement and precedes the ORDER BY clause. u.username in our example) must match the first expression used in ORDER BY clause. SELECT DISTINCT ON with ORDER BY The undecorated SELECT DISTINCT ON selects one row for each group but you don't know which of the rows will be selected. with res as ( select x, y from t1 union all select x, y from t2 ) select distinct on (x) x, y from res order by x, y desc Note: The leftmost expression in the ORDER BY clause must match the DISTINCT ON expression. To sort the rows of the result set, you use the ORDER BY clause in the SELECT statement. On Tue, 12 Feb 2019, Jeff Ross wrote: > Try (select (max(A.next_contact) from A) Thanks, Jeff. If you are in PostgreSQL however you can use the much simpler to write DISTINCT ON construct - like so SELECT DISTINCT ON (c.customer_id) c.customer_id, c.customer_name, o.order_date, o.order_amount, o.order_id FROM customers c LEFT JOIN orders O ON c.customer_id = o.customer_id ORDER BY c.customer_id, o.order_date DESC, o.order_id DESC; SELECT DISTINCT ON (address_id) LAST_VALUE(purchases.address_id) OVER wnd AS address_id FROM "purchases" WHERE "purchases". Each execution of the query can return different rows. And the reason I haven't heard about it is: Nonstandard Clauses DISTINCT ON ( … ) is an extension of the SQL standard. DISTINCT ON requires that its expression (i.e. 19. The DISTINCT ON expressions are interpreted using the same rules as for ORDER … Let's look at the simplest DISTINCT clause example in PostgreSQL. SELECT DISTINCT column_name1 FROM table_name; Explanation: In order to evaluate the duplicate rows, we use the values from the column_name1 column. For other columns, it will return the corresponding values. Making use of a non-leading column of an index. select_statement EXCEPT [ ALL | DISTINCT ] select_statement. If we continuously perform the ORDER BY clause with the DISTINCT ON (expression) to make the outcome expectable as it is an excellent exercise to perform. PostgreSQL does all the heavy lifting for us. SQL> select distinct x, y from test order by 1,2; x | y -----+--- lion | 1. lion | 2. rabbit | 1. rabbit | 2. tiger | 1. tiger | 2 (6 rows)-- PostgreSQL 에는 distinct on 이라는 고유한 syntax 가 있습니다. A list of the average test scores for each department clause must match the expression... You use the values from the SELECT statement returns rows in an unspecified ORDER multiple use... Table, the SELECT query and only display one unique row from result set scores for department... Will add the DISTINCT ON, we use the ORDER BY clause in a SELECT statement index. 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