The weather class can be queried with normal relational selection and projection queries. A SQL SELECT statement is used to do this. The statement is divided into a target list (the part that lists the attributes to be returned) and a qualification (the part that specifies any restrictions). For example, to retrieve all the rows of weather, type:
SELECT * FROM weather;
and the output should be:
+--------------+---------+---------+------+------------+
|city | temp_lo | temp_hi | prcp | date |
+--------------+---------+---------+------+------------+
|San Francisco | 46 | 50 | 0.25 | 11-27-1994 |
+--------------+---------+---------+------+------------+
|San Francisco | 43 | 57 | 0 | 11-29-1994 |
+--------------+---------+---------+------+------------+
|Hayward | 37 | 54 | | 11-29-1994 |
+--------------+---------+---------+------+------------+
You may specify any arbitrary expressions in the target list.
For example, you can do:
SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather;
Arbitrary Boolean operators (AND, OR and NOT) are allowed in the qualification of any query. For example,
SELECT * FROM weather
WHERE city = 'San Francisco'
AND prcp > 0.0;
results in:
+--------------+---------+---------+------+------------+
|city | temp_lo | temp_hi | prcp | date |
+--------------+---------+---------+------+------------+
|San Francisco | 46 | 50 | 0.25 | 11-27-1994 |
+--------------+---------+---------+------+------------+
As a final note, you can specify that the results of a select can be returned in a sorted order or with duplicate instances removed.
SELECT DISTINCT city
FROM weather
ORDER BY city;