Learning MySQL By Example

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

5.7. 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

7.1. SQL View Explained

7.2. Benefits of Using Views

7.3. Views That Allow UPDATE Statements

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

5.6

COUNT(column_name) and COUNT(*)

How They Are Different

COUNT(column_name) and COUNT(*)

COUNT(*) is the only aggregate function that counts rows with null values.
When you specify a count based on a specific column, null values will not be counted.

Code Sample:

USE bike; SELECT COUNT(phone), COUNT(*) FROM CUSTOMER

Output:

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