The self-join can be extended to display the number of days elapsed at each date : Self Join « Join « SQL / MySQL

1. SQL / MySQL
2. Join
3. Self Join

The self-join can be extended to display the number of days elapsed at each date

    
mysql>
mysql> CREATE TABLE rainfall
    -> (
    ->  date    DATE NOT NULL,
    ->  precip  FLOAT(10,2) NOT NULL
    -> );
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO rainfall (date, precip)
    ->  VALUES
    ->          ('2002-06-01', 1.5),
    ->          ('2002-06-02', 0),
    ->          ('2002-06-03', 0.5),
    ->          ('2002-06-04', 0),
    ->          ('2002-06-05', 1.0)
    -> ;
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql>
mysql> SELECT * FROM rainfall;
+------------+--------+
| date       | precip |
+------------+--------+
| 2002-06-01 |   1.50 |
| 2002-06-02 |   0.00 |
| 2002-06-03 |   0.50 |
| 2002-06-04 |   0.00 |
| 2002-06-05 |   1.00 |
+------------+--------+
5 rows in set (0.00 sec)

mysql>
mysql> SELECT t1.date, t1.precip AS 'daily precip',
    -> SUM(t2.precip) AS 'cum. precip',
    -> COUNT(t2.precip) AS days,
    -> AVG(t2.precip) AS 'avg. precip'
    -> FROM rainfall AS t1, rainfall AS t2
    -> WHERE t1.date >= t2.date
    -> GROUP BY t1.date;
+------------+--------------+-------------+------+-------------+
| date       | daily precip | cum. precip | days | avg. precip |
+------------+--------------+-------------+------+-------------+
| 2002-06-01 |         1.50 |        1.50 |    1 |    1.500000 |
| 2002-06-02 |         0.00 |        1.50 |    2 |    0.750000 |
| 2002-06-03 |         0.50 |        2.00 |    3 |    0.666667 |
| 2002-06-04 |         0.00 |        2.00 |    4 |    0.500000 |
| 2002-06-05 |         1.00 |        3.00 |    5 |    0.600000 |
+------------+--------------+-------------+------+-------------+
5 rows in set (0.00 sec)

mysql>
mysql> drop table rainfall;
Query OK, 0 rows affected (0.00 sec)

   
    
    
    
  

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