In SQL, COUNT(*) and COUNT(1) are often used interchangeably to count the number of rows in a table. While they achieve the same result, there are subtle differences in their usage and performance. Here’s a detailed comparison:-
COUNT(*)
- Usage: Counts all rows in the table, including rows with NULL values.
- Syntax:
SELECT COUNT(*) FROM table_name;
- Performance: Modern SQL databases optimize COUNT(*) queries efficiently. The * does not mean that all columns are selected; it is simply a standard notation to count all rows.
COUNT(1)
- Usage: Counts all rows in the table. The 1 is a constant expression and does not refer to any column.
- Syntax:
SELECT COUNT(1) FROM table_name;
- Performance: Similar to COUNT(*), modern SQL databases optimize COUNT(1) effectively. Since 1 is a constant, it can be slightly more efficient in certain scenarios because it doesn’t need to reference any columns.
Differences:-
1. Conceptual Difference:-
- COUNT(*) counts all rows, considering all columns.
- COUNT(1) counts all rows, but 1 is just a placeholder and doesn’t reference any column.
2. Performance:-
- In most modern SQL databases, there is no significant performance difference between COUNT(*) and COUNT(1) due to query optimization techniques.
- Older versions of some databases might show slight differences, but these differences are generally negligible.
3. Usage in Joins:
- When using joins, COUNT(*) can be useful to count rows from a specific table even if some columns are joined with NULL values.
- COUNT(1) remains unaffected by the actual columns being joined, as it uses a constant.
Example:-
Given a table employees:
| id | name | department |
|----|----------|------------|
| 1 | John Doe | Sales |
| 2 | Jane Doe | Marketing |
| 3 | Alice | NULL |
Using COUNT(*):-
SELECT COUNT(*) FROM employees;
Result: 3
Using COUNT(1):
SELECT COUNT(1) FROM employees;
Result: 3
In both cases, the result is the same: 3, indicating that there are three rows in the employees table
For most practical purposes, COUNT(*) and COUNT(1) are interchangeable and will produce the same results with similar performance. The choice between them is often a matter of personal or organizational coding standards rather than a strict performance or functionality requirement.
