8.4. LEFT OUTER JOIN
- The MySQL LEFT JOIN clause allows you to query data from two or more database tables. The LEFT JOIN clause is an optional part of the SELECT statement, which appears after the FROM clause.
- When you join the t1 table to the t2 table using the LEFT JOIN clause, if a row from the left table t1matches a row from the right table t2 based on the join condition ( t1.c1 = t2.c1 ), this row will be included in the result set.
- In case the row in the left table does not match with the row in the right table, the row in the left table is also selected and combined with a “fake” row from the right table. The fake row contains NULL for all corresponding columns in the SELECT clause.
- In other words, the LEFT JOIN clause allows you to select rows from the both left and right tables that are matched, plus all rows from the left table ( t1 ) even with no matching rows found in the right table ( t2 ).
- The following Venn diagram helps you visualize how the LEFT JOIN clause works. The intersection between two circles are rows that match in both tables, and the remaining part of the left circle are rows in the t1 table that do not have any matching row in the t2 table. Hence, all rows in the left table are included in the result set.
QUERY: - SELECT T1.column_name, T2.column_name FROM table1 T1 LEFT JOIN table1 T2 on condition;
EXAMPLE: - SELECT a.e_id, a.f_name, b.salary FROM employee a LEFT JOIN salary b on a.e_id = b.e_id;
Last modified: Monday, 2 December 2019, 12:19 PM