SQL Joins Cheat Sheet

1. **Quick Reference**

| Join Type | Description |

| --- | --- |

| INNER JOIN | Returns records with matching values in both tables |

| LEFT JOIN | Returns all records from the left table and matching records from the right table |

| RIGHT JOIN | Returns all records from the right table and matching records from the left table |

| FULL OUTER JOIN | Returns all records from both tables with null values where no match exists |

2. **Syntax**

The basic syntax for a SQL join is:

SELECT *
FROM table1
JOIN table2
ON table1.column_name = table2.column_name;

* table1 and table2 are the names of the tables to be joined

* column_name is the name of the column to join on

* JOIN can be replaced with the specific type of join (e.g. INNER JOIN, LEFT JOIN, etc.)

3. **Common Patterns**

* Inner Join with Multiple Conditions:

SELECT *
FROM table1
INNER JOIN table2
ON table1.column1 = table2.column1 AND table1.column2 = table2.column2;

* Left Join with Null Check:

SELECT *
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name
WHERE table2.column_name IS NULL;

* Full Outer Join with Order By:

SELECT *
FROM table1
FULL OUTER JOIN table2
ON table1.column_name = table2.column_name
ORDER BY table1.column_name;

* Joining Multiple Tables:

SELECT *
FROM table1
INNER JOIN table2
ON table1.column_name = table2.column_name
INNER JOIN table3
ON table2.column_name = table3.column_name;

4. **Gotchas**

* No Matching Records: If there are no matching records in the joined tables, the result set will be empty.

* Null Values: If a join is performed on a column that contains null values, the result set may not be what is expected.

* Table Order: The order of the tables in a join can affect the result set, especially when using LEFT JOIN or RIGHT JOIN.

* Joining on Non-Unique Columns: Joining on non-unique columns can result in a Cartesian product, which can be very large and slow.

5. **Related Commands**

* UNION and UNION ALL for combining the results of multiple SELECT statements

* GROUP BY and HAVING for grouping and filtering the results of a join

* SUBQUERY for using the results of one query as input to another query

* EXISTS and IN for checking if a record exists in a table or if a value is in a list of values.