What is the function of the COUNT command in SQL?

The COUNT command in SQL is used to return the number of rows that matches a specified criterion.

In more detail, the COUNT command is an aggregate function in SQL that provides the number of rows in a table that satisfy the conditions of the SQL query. It is a very useful command when you need to know the number of records in a table that match certain criteria.

For example, if you have a table of students and you want to know how many students are in Year 13, you could use the COUNT command in your SQL query to find this out. The command would look something like this: SELECT COUNT(*) FROM students WHERE year = 13. This command would return the number of students in Year 13.

The COUNT command can be used with or without the WHERE clause. If used without the WHERE clause, it will simply return the total number of rows in the table. For example, SELECT COUNT(*) FROM students would return the total number of students.

There are three forms of the COUNT function: COUNT(*), COUNT(expression) and COUNT(DISTINCT expression). COUNT(*) counts all rows in the target table whether they contain null values or not. COUNT(expression) counts only the rows where the expression is not null. COUNT(DISTINCT expression) counts only the distinct (different) non-null values.

In summary, the COUNT command in SQL is a powerful tool for analysing data in a database. It allows you to quickly and easily determine the number of rows that match specific criteria, which can be invaluable when dealing with large datasets.