SQL Joins are used to combine data from multiple tables in a single query. They are an important part of any SQL query and allow you to select, update, and delete data from multiple tables based on relationships between the tables. There are several types of SQL Joins, including INNER JOIN, OUTER JOIN, and CROSS JOIN.
The INNER JOIN is the most common type of join and is used to select rows from both tables that have a matching column value. It is also known as an EQUIJOIN. Here is the syntax:
SELECT column1, column2, ...
FROM table1
INNER JOIN table2 ON table1.column = table2.column;
The OUTER JOIN is used to select rows from both tables, even if there is no matching value in the other table. There are three types of OUTER JOINS: LEFT JOIN, RIGHT JOIN, and FULL JOIN. Here is the syntax for a LEFT JOIN:
SELECT column1, column2, ...
FROM table1
LEFT JOIN table2 ON table1.column = table2.column;
The RIGHT JOIN and FULL JOIN have the same syntax, but the order of the tables is reversed:
SELECT column1, column2, ...
FROM table2
RIGHT JOIN table1 ON table1.column = table2.column;
SELECT column1, column2, ...
FROM table1
FULL JOIN table2 ON table1.column = table2.column;
The CROSS JOIN is used to select all rows from both tables, regardless of whether there is a matching value or not. It is also known as a CARTESIAN JOIN. Here is the syntax:
SELECT column1, column2, ...
FROM table1
CROSS JOIN table2;
You can use these joins in combination with the WHERE clause, GROUP BY, and other clauses to create complex queries. You can learn more about these and other advanced topics in our SQL tutorials and references.