In SQL, you can use the COUNT() function to count the number of rows that match a specific condition. Here's the basic syntax:

SELECT COUNT(column_name)
FROM table_name
WHERE condition;

You can replace column_name with the name of the column you want to count the values of, and table_name with the name of the table you want to count the values from. The WHERE clause is optional and can be used to specify a condition that the rows must meet in order to be counted.

Here's an example:

SELECT COUNT(*)
FROM orders
WHERE status = 'completed';

This query will count the number of rows in the orders table where the status column is equal to 'completed'. The * inside the COUNT() function means that it will count all rows that match the condition, regardless of the values in the columns.

You can also use COUNT(DISTINCT column_name) to count the number of unique values in a column:

SELECT COUNT(DISTINCT customer_id)
FROM orders;

This query will count the number of unique values in the customer_id column of the orders table.