![]() ![]() For example, you can represent an inner join as a Cartesian product that retains only the rows that satisfy the join condition. SQL does not offer explicit semijoin or antijoin operations, but some expressions ( EXISTS and NOT EXISTS, for example) can be used to achieve equivalent results.Īll of the above are logical operations. ![]() The antijoin includes only those rows from the first set that didn't have a matching pair in the second set. Secondly, it includes the rows from the first set only once, even if a row happens to have multiple matches in the second set. Firstly, it includes only the rows from the first set that have a matching pair in the second set. The semijoin is similar to a regular inner join, but with two key differences. It includes rows with missing pairs from both sets, as well as the INNER JOIN result. The FULL JOIN is the LEFT JOIN and the RIGHT JOIN combined. The RIGHT JOIN works identically, except the joining order is reversed. ![]() The LEFT OUTER JOIN (or the LEFT JOIN) supplements the result of an inner join with the rows from the left set which didn't have a corresponding pair in the right set (the contents of the missing right set columns are set to NULLs). This is a specific case of an inner join with a TRUE condition. If the join condition is an equality operator, as is often the case, then the join is called an equijoin.Ī Cartesian product (or a CROSS JOIN) of two sets includes all possible combinations of pairs of rows from both sets. All the columns involved comprise what is known as the join key. A join condition is what ties together columns from the first row set with columns from the second one. An INNER JOIN (or usually just JOIN) is a subset of two sets that includes all the row pairs from the two original sets that match the join condition. Sets of rows (whether pulled directly from a table or formed as a result of an operation) are always joined together in pairs. Joins are the primary feature of SQL, the foundation that enables its power and agility. Additionally, we will check out the row memoization feature introduced in PostgreSQL 14. This article will remind you what logical join types are out there, and then discuss one of three physical join methods, the Nested loop join. The next item on the list is join methods. So far we've discussed query execution stages, statistics, and the two basic data access methods: Sequential scan and Index scan. ![]()
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