2.5. Querying a Table

To retrieve data from a table, the table is queried. An SQL SELECT statement is used to do this. The statement is divided into a select list (the part that lists the columns to be returned), a table list (the part that lists the tables from which to retrieve the data), and an optional qualification (the part that specifies any restrictions). For example, to retrieve all the rows of table weather, type:

SELECT * FROM weather;

(here * means "all columns") and the output should be:

 city | temp_lo | temp_hi | prcp | date
---------------+---------+---------+------+------------
 San Francisco | 46 | 50 | 0.25 | 1994年11月27日
 San Francisco | 43 | 57 | 0 | 1994年11月29日
 Hayward | 37 | 54 | | 1994年11月29日
(3 rows)

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;

This should give:

 city | temp_avg | date
---------------+----------+------------
 San Francisco | 48 | 1994年11月27日
 San Francisco | 50 | 1994年11月29日
 Hayward | 45 | 1994年11月29日
(3 rows)

Notice how the AS clause is used to relabel the output column. (It is optional.)

Arbitrary Boolean operators (AND, OR, and NOT) are allowed in the qualification of a query. For example, the following retrieves the weather of San Francisco on rainy days:

SELECT * FROM weather
 WHERE city = 'San Francisco'
 AND prcp > 0.0;

Result:

 city | temp_lo | temp_hi | prcp | date
---------------+---------+---------+------+------------
 San Francisco | 46 | 50 | 0.25 | 1994年11月27日
(1 row)

As a final note, you can request that the results of a select can be returned in sorted order or with duplicate rows removed. (Just to make sure the following won't confuse you, DISTINCT and ORDER BY can be used separately.)

SELECT DISTINCT city
 FROM weather
 ORDER BY city;

 city
---------------
 Hayward
 San Francisco
(2 rows)