COUNT(expr) doesn't count NULL values is useful when producing multiple counts from the same set of values.
mysql>
mysql> CREATE TABLE mytable
-> (
-> rec_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
-> name VARCHAR(20) NOT NULL,
-> trav_date DATE NOT NULL,
-> miles INT NOT NULL,
-> PRIMARY KEY (rec_id)
-> );
Query OK, 0 rows affected (0.00 sec)
mysql>
mysql> INSERT INTO mytable (name,trav_date,miles)
-> VALUES
-> ('Ben','2010-11-30',152),
-> ('Suzi','2010-11-29',391),
-> ('Henry','2010-11-29',300),
-> ('Henry','2010-11-27',96),
-> ('Ben','2010-11-29',131),
-> ('Henry','2010-11-26',115),
-> ('Suzi','2010-12-02',502),
-> ('Henry','2010-12-01',197),
-> ('Ben','2010-12-02',79),
-> ('Henry','2010-11-30',203)
-> ;
Query OK, 10 rows affected (0.00 sec)
Records: 10 Duplicates: 0 Warnings: 0
mysql>
mysql> SELECT
-> COUNT(IF(DAYOFWEEK(trav_date)=7,1,NULL)) AS 'Saturday trips',
-> COUNT(IF(DAYOFWEEK(trav_date)=1,1,NULL)) AS 'Sunday trips'
-> FROM mytable;
+----------------+--------------+
| Saturday trips | Sunday trips |
+----------------+--------------+
| 1 | 0 |
+----------------+--------------+
1 row in set (0.00 sec)
mysql>
mysql>
mysql> drop table mytable;
Query OK, 0 rows affected (0.00 sec)
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