This artical describes the very basics of joins.
- By using joins, we can retrieve data from two or more tables based on logical relationships between the tables. It combines the fields vertically.
- Join conditions can be specified in either the FROM or WHERE clauses; specifying them in the FROM clause is recommended. WHERE and HAVING clauses can also contain search conditions to further filter the rows selected by the join conditions.
SELECT WhatColumn(s) FROM ChildTable TypeOfJoin ParentTable ON Condition
- The Cartesian product of the tables
- Conditions eliminate unwanted rows from the results.
Joins can be categorized as:
- Inner Join
- Outer Join
- Full Join
- Cross Join
- Self Join
INNER JOIN – Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table the typical join operation, which uses some comparison operator like = or <>). These are called as equi-joins or non equi-joins.
OUTER JOIN - Outer joins can be a left, a right, or full outer join.Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause
LEFT JOIN or LEFT OUTER JOIN
- The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match.
- When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.
RIGHT JOIN or RIGHT LEFT OUTER JOIN
- A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.
FULL JOIN or FULL OUTER JOIN
A full outer join returns all rows in both the left and right tables. Any time a row has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables, the entire result set row contains data values from the base tables.
Cross joins return all rows from the left table, each row from the left table is combined with all rows from the right table. Cross joins are also called Cartesian products.
A table joining itself called as Self Join.