what is – Page 146

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.

SQL SUM() for Beginners

July 23, 2023December 8, 2020 by Ian

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

It can also be used to return the sum 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.

SQL MAX() for Beginners

July 23, 2023December 7, 2020 by Ian

In SQL, the MAX() function is an aggregate function that returns the maximum value in a given expression.

Below are some basic examples to demonstrate how it works.

SQL MIN() for Beginners

July 23, 2023December 7, 2020 by Ian

In SQL, the MIN() function is an aggregate function that returns the minimum value in a given expression.

Below are some basic examples to demonstrate how it works.

SQL COUNT() for Beginners

July 23, 2023December 7, 2020 by Ian

In SQL, the COUNT() function is an aggregate function that returns the number of items found in a group.

You can use COUNT() in multiple parts of a query. For example, you can use it in the SELECT list, or the HAVING clause when filtering groups.

SQL HAVING Clause for Beginners

December 7, 2020December 6, 2020 by Ian

In SQL, the HAVING clause can be used to specify a search condition for a group or an aggregate.

The HAVING clause is typically used with the GROUP BY clause. In cases where it’s not, there is an implicit single, aggregated group.

SQL SELECT for Beginners

September 17, 2024December 6, 2020 by Ian

The SELECT statement is almost without doubt, the most commonly used statement in SQL.

The SELECT statement is used for retrieving data from the database. You can specify which rows you want returned, and which columns.

SQL GROUP BY Clause for Beginners

December 7, 2020December 5, 2020 by Ian

In SQL, the GROUP BY clause can be used to divide the results of a query into groups of rows.

This is usually done in order to perform one or more aggregations on each group.