sql – Page 2

Create a Relationship in SQL

February 18, 2021February 16, 2021 by Ian

In SQL, you create a relationship by creating a foreign key constraint.

More specifically, you have a parent table and a child table. The parent contains the primary key, and the child table contains a foreign key that references the primary key of the parent table.

When you use SQL to create a relationship, you can create the relationship at the time you create the table, or you can create it later (by altering the table). This article covers both scenarios.

SQL Reference for Beginners

February 18, 2021December 12, 2020 by Ian

This page contains direct links to SQL tutorials and reference articles for beginners.

How to Rename a Table in SQL

December 11, 2020 by Ian

In SQL, most RDBMSs allow you to rename a table using the ALTER TABLE statement, which is the statement used to modify the definition of an existing table.

Some RDBMs also include a RENAME statement for renaming tables.

But if you use SQL Server, you’ll need to use the sp_rename stored procedure.

SQL DROP TABLE for Beginners

November 15, 2025December 11, 2020 by Ian

In SQL, if you want to remove a table from a database, you need to use the DROP TABLE statement.

That destroys the table and all its data.

SQL DROP COLUMN for Beginners

December 10, 2020 by Ian

In SQL, if you want to remove a column from a table, you need to use the ALTER TABLE statement with the DROP COLUMN clause.

That removes the column and all its data.

SQL Tutorial for Beginners

February 18, 2021December 9, 2020 by Ian

In this SQL tutorial for beginners, you will create your own database, insert data into that database, and then run queries against that database.

This SQL tutorial will get you running SQL queries in no time!

How to Rename a Column in SQL

December 9, 2020 by Ian

This article provides a quick example of renaming a table in SQL.

Most of the major RDBMSs allow you to rename a column with the ALTER TABLE statement. SQL Server is an exception.

SQL ALTER TABLE for Beginners

December 9, 2020December 9, 2020 by Ian

In SQL, the ALTER TABLE statement modifies the definition of an existing table.

You can use ALTER TABLE to alter, add, or drop columns and constraints.

Depending on your DBMS, the ALTER TABLE statement can also be used to reassign and rebuild partitions, or disable and enable constraints and triggers.

SQL UNION Clause for Beginners

October 22, 2023December 8, 2020 by Ian

In SQL, the UNION clause concatenates the results of two queries into a single result set.

You can use the UNION clause with or without the ALL argument:

UNION ALL – Includes duplicates.
UNION – Excludes duplicates.

Some RDBMSs also accept UNION DISTINCT, which is the equivalent to UNION. That is, it excludes duplicates.

Below are some basic examples to demonstrate how it works.

SQL AVG() for Beginners

July 23, 2023December 8, 2020 by Ian

In SQL, the AVG() function is an aggregate function that returns the average of all values in a given expression.

It can also be used to return the average of all distinct (unique) values in an expression.

The expression must be numeric (it cannot be character string, bit string, or datetime).

Below are some basic examples to demonstrate how it works.