How to Use JOINS in PostgreSQL

Galaxy Glossary

How do I use JOINs in SQL Server?

JOINs combine rows from two or more tables based on a related column.

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Description

Example H2

Example H3

Why use JOINs instead of subqueries?

JOINs let the optimizer combine tables before filtering, often running faster and keeping code readable. Subqueries materialize intermediate results that can slow performance in large datasets.

Which JOIN types does SQL Server support?

SQL Server offers INNER, LEFT OUTER, RIGHT OUTER, FULL OUTER, CROSS, and SELF JOINs. Each dictates how unmatched rows are handled when combining tables.

How do I write an INNER JOIN?

Use INNER JOIN when you only need rows with matching keys in both tables. It is the default JOIN type when you omit the word INNER.

SELECT c.name, o.total_amount FROM Customers AS c INNER JOIN Orders AS o ON o.customer_id = c.id;

How do I include unmatched rows?

Use OUTER JOINs. LEFT returns all rows from the first (left) table, filling NULLs for non-matches. RIGHT does the opposite; FULL returns rows from both sides.

-- Customers without orders SELECT c.name, o.id AS order_id FROM Customers AS c LEFT JOIN Orders AS o ON o.customer_id = c.id;

When should I use CROSS JOIN?

CROSS JOIN creates a cartesian product. Use it for generating test data or pairing every customer with every product only when necessary, as row counts multiply quickly.

Best practices for JOINs?

1) Index foreign keys such as Orders.customer_id. 2) Always qualify column names to avoid ambiguity. 3) Filter early with ON, not WHERE, to minimize result size.

How do I JOIN more than two tables?

Chain JOIN clauses. SQL Server evaluates them left to right, applying ON conditions at each step.

SELECT c.name, p.name AS product, oi.quantity FROM Customers AS c JOIN Orders AS o ON o.customer_id = c.id JOIN OrderItems AS oi ON oi.order_id = o.id JOIN Products AS p ON p.id = oi.product_id;

How to debug a misbehaving JOIN?

Check ON conditions for wrong columns, verify data types match, and temporarily switch to FULL JOIN to see unmatched rows quickly.

Why How to Use JOINS in PostgreSQL is important

How to Use JOINS in PostgreSQL Example Usage


-- Average spend per customer, including those with no orders
SELECT c.id,
       c.name,
       AVG(o.total_amount) AS avg_spend
FROM   Customers AS c
LEFT JOIN Orders AS o
       ON o.customer_id = c.id
GROUP BY c.id, c.name;

How to Use JOINS in PostgreSQL Syntax


-- INNER JOIN
SELECT column_list
FROM   table1 [AS t1]
INNER JOIN table2 [AS t2]
       ON t1.common_col = t2.common_col;

-- LEFT | RIGHT | FULL OUTER JOIN
SELECT column_list
FROM   table1
LEFT  | RIGHT | FULL OUTER JOIN table2
       ON join_condition;

-- CROSS JOIN
SELECT column_list
FROM   table1
CROSS JOIN table2;

-- SELF JOIN (table joined to itself)
SELECT c1.name, c2.referred_by
FROM   Customers AS c1
JOIN   Customers AS c2
       ON c1.id = c2.referred_by;

Common Mistakes

Using WHERE for OUTER JOIN filters. Moving criteria from WHERE to the ON clause preserves unmatched rows, preventing accidental row loss.
Selecting * from multiple tables with same column names. Always prefix columns or alias them to avoid ambiguous-column errors and unintended overwrites.