"id" or "title" are typically not very distinctive and not very useful identifiers. I added aliases to your original (un-aggregated) query to clarify: SELECT i.id, i.title AS item_title, t.title AS tag_title Understanding multiple table join with aggregation.Why is array_agg() slower than the non-aggregate ARRAY() constructor?.Starting in C 11, the nint and nuint types are aliases. The long answer: CAST is an ANSI SQL standard that came about with SQL-92, meaning that it is portable across different Database Management Systems (DBMS) and will work the. What is the difference between LATERAL and a subquery in PostgreSQL? How to convert text to integer in SQL If table column is VARCHAR and has. The short answer: The SQL CAST function is used to explicitly convert a given data type to a different data type in a SQL database.Since an ARRAY constructor always produces a row (with empty array if the subquery is empty - subtle difference in the result!), LEFT JOIN LATERAL (.) ON true is not needed here. One option is to compute the cartesian product of your two tables, then extract the first two matching rows for each 't1.x' value that have the least difference of values with respect to 't2. Two SQL LEFT JOINS produce incorrect resultįor a small percentage of rows, use a LATERAL join with an ARRAY constructor: SELECT id, title AS item_title, t.tag_arrayįROM items i, LATERAL ( - this is an implicit CROSS JOIN.Joining before aggregation also gets out of hands with more than one 1:n table in the FROM list (not in this simple case). Since that does not multiply rows in the join, we need no GROUP BY in the outer SELECT. Use LEFT JOIN in the outer query if there can be items without tags - which would be excluded with JOIN. SELECT it.item_id AS id, array_agg(t.title) AS tag_array While querying all or most items, it is typically substantially faster to aggregate rows from the "many"-table first and join later: SELECT id, i.title AS item_title, t.tag_array
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