![]() ![]() SELECT city, temp_lo, temp_hi, prcp, date, location In practice this is undesirable, though, so you will probably want to list the output columns explicitly rather than using *: This is correct because the lists of columns from the weather and cities tables are concatenated. There are two columns containing the city name. We will see shortly how this can be fixed. This is because there is no matching entry in the cities table for Hayward, so the join ignores the unmatched rows in the weather table. There is no result row for the city of Hayward. SELECT * FROM weather JOIN cities ON city = name Ĭity | temp_lo | temp_hi | prcp | date | name | location This would be accomplished by the following query: For example, to return all the weather records together with the location of the associated city, the database needs to compare the city column of each row of the weather table with the name column of all rows in the cities table, and select the pairs of rows where these values match. They combine rows from one table with rows from a second table, with an expression specifying which rows are to be paired. Queries that access multiple tables (or multiple instances of the same table) at one time are called join queries. Queries can access multiple tables at once, or access the same table in such a way that multiple rows of the table are being processed at the same time. ![]() ![]() Thus far, our queries have only accessed one table at a time. ![]()
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