Table of Contents for

MySQL in a Nutshell, 2nd Edition

This function returns the number of the quarter (1–4) for the date provided. The first quarter (i.e., the first three months) of each year has a value of 1. Here is an example:

SELECT COUNT(appointment)
AS 'Appts. Last Quarter'
FROM appointments
WHERE QUARTER(appointment) = (QUARTER(NOW( )) - 1)
AND client_id = '7393';

+---------------------+
| Appts. Last Quarter |
+---------------------+
|                  16 |
+---------------------+

In this example, MySQL calculates the total number of appointments for a particular client that occurred before the current quarter. The flaw in this SQL statement is that it doesn’t work when it’s run during the first quarter of a year. In the first quarter, the calculation on the fourth line would produce a quarter value of 0. This statement also doesn’t consider appointments in previous quarters of previous years. To solve these problems, we could set up user-defined variables for the values of the previous quarter and for its year:

SET @LASTQTR:=IF((QUARTER(CURDATE( ))-1) = 0, 4, QUARTER(CURDATE( ))-1);

SET @YR:=IF(@LASTQTR = 4, YEAR(NOW( ))-1, YEAR(NOW( )));

SELECT COUNT(appointment) AS 'Appts. Last Quarter'
FROM appointments
WHERE QUARTER(appointment) = @LASTQTR
AND YEAR(appointment) = @YR
AND client_id = '7393';

In the first SQL statement here, we use an IF statement to test whether reducing the quarter by 1 would yield a 0 value. If so, we’ll set the user variable for the last quarter to 4. In the second statement, we establish the year for the last quarter based on the value determined for @LASTQTR. The last SQL statement selects rows and counts them where the QUARTER() function yields a value equal to the @LASTQTR variable and where the YEAR() function yields a value equal to the @YR variable based on the appointment date, and where the client is the one for which we are running the statement.