SQL quiz. Given an <b>Income</b> table which, among others,

Given an Income table which, among others, contains the following two columns and the corresponding data in them:

+--------+------+
| Profit | Loss |
+--------+------+
|   1000 |  100 |
|   2000 | NULL |
|   3000 | 1000 |
|   NULL |  500 |
|   NULL | NULL |
+--------+------+

What value will the following query return:

SELECT COUNT(Profit-Loss)
FROM Income;

Explanation

Arithmetic operators will always return NULL if at least one of its arguments is NULL.
Aggregate functions simply ignore NULL values.
Therefore, 3 values out of 5 will be ignored by COUNT() function.

Theory

SQL Aggregate Functions
SQL aggregate functions return a single value, calculated from values in a column.
Useful aggregate functions:
- AVG() - Returns the average value
- COUNT() - Returns the number of rows
- FIRST() - Returns the first value
- LAST() - Returns the last value
- MAX() - Returns the largest value
- MIN() - Returns the smallest value
- SUM() - Returns the sum
Read more: SQL Functions
COUNT() Function
The COUNT() function returns the number of rows that matches a specified criteria.
SQL COUNT(column_name) Syntax
The COUNT(column_name) function returns the number of values (NULL values will not be counted) of the specified column:
```
SELECT COUNT(column_name) FROM table_name;
```
SQL COUNT(*) Syntax
The COUNT(*) function returns the number of records in a table:
```
SELECT COUNT(*) FROM table_name;
```
The COUNT(DISTINCT column_name) function returns the number of distinct values of the specified column:
SQL COUNT(DISTINCT column_name) Syntax
```
SELECT COUNT(DISTINCT column_name) FROM table_name;
```
Note: COUNT(DISTINCT) works with ORACLE and Microsoft SQL Server, but not with Microsoft Access.
Read more.