JOIN clause

Use the JOIN clause to join data from different tables together based on logical relationships.

• Syntax
• Join types
  • INNER JOIN
  • LEFT [OUTER] JOIN
  • RIGHT [OUTER] JOIN
  • FULL [OUTER] JOIN
• Troubleshoot joins

Syntax

SELECT_clause
FROM <left_join_items>
[INNER | LEFT [OUTER] | RIGHT [OUTER] | FULL [OUTER]] JOIN <right_join_items>
ON <join_condition>
[WHERE_clause]
[GROUP_BY_clause]
[HAVING_clause]
[ORDER_BY_clause]

Arguments

• left_join_items: One or more tables specified in the FROM clause that represent the left side of the join.
• right_join_items: One or more tables specified in the JOIN clause that represent the right side of the join.
• join_condition: A predicate expression in the ON clause that uses the = (equal to) comparison operator to compare column values from the left side of the join to column values on the right side of the join. Rows with values that match the defined predicate are joined using the specified join type.

Join types

The following joins types are supported:

Join sample tables

The examples below illustrate join methods using the following tables:

INNER JOIN

Inner joins combine rows from tables on the left and right side of the join based on common column values defined in the ON clause. Rows that don’t have matching column values are not included in the output table.

SELECT
  *
FROM
  prod_line
RIGHT JOIN errors ON
  prod_line.time = errors.time
  AND prod_line.station = errors.station
ORDER BY
  prod_line.time

LEFT [OUTER] JOIN

A left outer join returns all rows from the left side of the join and only returns data from the right side of the join in rows with matching column values defined in the ON clause.

SELECT
  *
FROM
  prod_line
LEFT JOIN errors ON
  prod_line.time = errors.time
  AND prod_line.station = errors.station
ORDER BY
  prod_line.time

RIGHT [OUTER] JOIN

A right outer join returns all rows from the right side of the join and only returns data from the left side of the join in rows with matching column values defined in the ON clause.

SELECT
  *
FROM
  prod_line
RIGHT JOIN errors ON
  prod_line.time = errors.time
  AND prod_line.station = errors.station
ORDER BY
  prod_line.time

FULL [OUTER] JOIN

A full outer join returns all data from the left and right sides of the join and combines rows with matching column values defined in the ON clause. Data that is not available on each respective side of the join is NULL.

SELECT
  *
FROM
  prod_line
FULL JOIN errors ON
  prod_line.time = errors.time
  AND prod_line.station = errors.station
ORDER BY
  time

Troubleshoot joins

Ambiguous reference to unqualified field

If a column exists on both sides of the join and is used in in the SELECT, ON, WHERE, HAVING, GROUP BY, or ORDER BY clause, you must use a fully-qualified reference. For example, if both sides of the join have a time column and you want to explicitly select a time column, you must specifiy which side of the join to use the time column from:

SELECT
  prod_line.time,
  produced,
  message,
FROM
  prod_line
INNER JOIN errors ON
  -- ...