course_sql

SQL 3 - Joining data

Outcomes

Join together two or more tables using a key

Describe when to use each join type
Join tables with INNER JOIN
Join tables with LEFT OUTER JOIN
- Filter on NULL to find only rows without a match
Rename fields with AS

Videos:

INNER JOIN

Get all matching records in two tables.

SELECT * FROM tableA INNER JOIN tableB ON tableA.foreign_key = tableB.primary_key

SQL INNER JOIN is used to combine rows from two or more tables based on a related column between them.

Specify Columns and Tables: Begin the SELECT statement by specifying the columns you want to retrieve and the main table (table1) you want to query data from.
INNER JOIN: Use the INNER JOIN keyword to indicate that you want to combine data from another table (table2) based on a common column between them.
ON Clause: After the INNER JOIN keyword, use the ON keyword followed by a condition that defines the relationship between the two tables. The condition specifies the common column in both tables that will be used to match rows.

Example:

We have two tables: “employees” and “departments.” To retrieve a list of employees along with their corresponding department names:

SELECT employees.name, departments.department_name FROM employees INNER JOIN departments ON employees.department_id = departments.id;

Remember that for the INNER JOIN to work correctly, there should be a matching relationship between the columns specified in the ON clause. If there are no matches for a row in either table, that row will not be included in the result set.

Important! We sometimes use COUNT(*) to count the number of rows, and other times we use COUNT(field_name) to count a specific field. This is particularly important when we start using joins. If you select COUNT(*) and an outer join, you’ll get a different result than COUNT(field_that_may_be_null_due_to_a_join).

OUTER JOIN

Get all records from the left table, and all matching from the right.

SELECT tableA.field, tableB.* FROM tableA LEFT OUTER JOIN tableB ON tableA.foreign_key = tableB.primary_key

An outer join is the same as an inner join, with one important difference:

Inner join returns rows where the condition is true
Outer join returns all records from the table in FROM, and only those in the LEFT OUTER JOIN that match.

Rename

We often rename fields or tables with AS when doing joins. This is because we often have fields with matching names in between the two tables.

Further resources

See SQLZoo’s section on joins:

Problems

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