What is a Query

A query is a question about your data that allows you to retrieve specific information from your database. It is an instruction to the database system to search for a particular set of data that is of interest to you, and to present it in a useful format.

A query is written as an SQL SELECT statement, as in the following example of a fairly complex query that finds the number of customers in each city in New York or California that has at least one customer. The result is sorted by state and city.

SELECT   St, City, Count(C_no)
FROM     Customer
WHERE    St="CA"  OR  City="New York"  
GROUP BY St, City
HAVING   count(*) > 0
ORDER BY St, City

The SELECT statement consists of several important clauses.

• SELECT Clause

  Lists the columns to be retrieved from the table(s).

• FROM Clause

  Lists the tables from which data is to be retrieved.

• WHERE Clause

  Consists of a set of conditions that the rows of data must satisfy.

• GROUP BY Clause

  Organizes data into subsets or groups based on the values in the grouped columns. It is frequently used to create summary rows containing totals, sums, counts, averages, minimum values, and maximum values.

• HAVING Clause

  Defines conditions that summary rows of data must satisfy.

• ORDER BY Clause

  Specifies the order in which the rows of data should be sorted.