Learning MySQL By Example
Cover
Introduction
1. How to Retrieve Data From a Single Table
1.1. The Five Clauses of the SELECT Statement
1.2. Column Specifications
1.3. LIKE and REGEXP Operators
1.4. Arithmetic Operators
1.5. Column Aliases
1.6. Comparison Operators
1.7. IS NULL, BETWEEN, IN Operators
1.8. AND, OR, NOT Logical Operators
1.9. DISTINCT Clause
2. How to Retrieve Data from Multiple Tables
2.1. The JOIN Clause
2.2. Joining More Than Two Tables
2.3. The OUTER JOIN Clause
2.4. How to Code a UNION
3. Using Functions
3.1. Date Functions
3.2. Numeric Functions
3.3. String Functions
4. How to Insert, Update, Delete Data in Tables
4.1. The INSERT Clause With a Column List
4.2. The INSERT Clause Without a Column List
4.4. The UPDATE Clause With a Column List
4.4. The DELETE Clause
5. Summary Queries and Aggregate Functions
5.1. Aggregate Functions
5.2. Grouping Data
5.3. Simple GROUP BY Query
5.4. Improving the GROUP BY Query
5.5. Using the HAVING Clause
5.5. Using the HAVING and WHERE Clauses Together
5.6. COUNT(column_name) and COUNT(*)
5.7. Using the DISTINCT Statement
6. Working With Subqueries
6.1. The Subquery In a SELECT Statement
6.2. The Subquery in an UPDATE statement
6.3. Create a Duplicate Table From An Existing Table
6.4. The Subquery In a Delete Statement
7. SQL Views
7.1. SQL View Explained
7.2. Benefits of Using Views
7.3. Views That Allow UPDATE Statements
8. SQL Indexes
8.1. SQL Indexes Explained
8.2. Clustered vs. Non-clustered Indexes
8.3. Create an Index in Workbench Using an ERD
8.4. How to Manually Add an Index to an Existing Table
Glossary
Index
5.1

Aggregate Functions

Aggregate Functions

  • Aggregate functions are synonymous with column functions.
  • A summary query uses at least on column function.
  • AVG, SUM return numeric values.
  • MIN, MAX, COUNT can return numeric, date, or string values
  • All values are included in aggregate functions by default unless you specify the DISTINCT keyword
  • Duplicate rows are excluded in all aggregate functions with the exception of COUNT(*)
  • ***** IF YOU CODE AN AGGREGATE FUNCTION IN THE SELECT STATEMENT, YOU CANNOT ALSO INCLUDE NON-AGGREGATE FUNCTIONS IN THE SELECT STATEMENT UNLESS THOSE NON-AGGREGATE COLUMNS ARE INCLUDED IN A GROUP BY CLAUSE

Table 1. Aggregate Functions List

Aggregate Function

Output data-type

Result

AVG([DISTINCT] column_values)

numeric

The average of the non-null columns in the expression

SUM([DISTINCT] column_values)

numeric

The total of the non-null columns in the expression

MIN([DISTINCT] column_values)

numeric, date, string

The lowest value off the non-null columns in the expression

MAX([DISTINCT] column_values)

numeric, date, string

The highest value of the non-null columns in the expression

COUNT([DISTINCT] column_values)

numeric

The number of the non-null columns in the expression

COUNT(*)

numeric

The number of rows returned by the query

Code Sample:

USE bike;
SELECT AVG(list_price), SUM(list_price), MIN(list_price), 
    MAX(list_price), COUNT(list_price), COUNT(*)
FROM product;

Output:

ag_01.png

CC BY-NC-ND International 4.0

CC BY-NC-ND International 4.0: This work is released under a CC BY-NC-ND International 4.0 license, which means that you are free to do with it as you please as long as you (1) properly attribute it, (2) do not use it for commercial gain, and (3) do not create derivative works.

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