Difference between revisions of "Retrieving Data From a MySQL Database"

Revision as of 18:46, 9 October 2007 (view source) Neil (Talk | contribs) (→‎Using SELECT to Retrieve Mutiple Columns) ← Older edit		Revision as of 18:52, 9 October 2007 (view source) Neil (Talk | contribs) (→‎Using SELECT to Retrieve Mutiple Columns) Newer edit →
Line 50:		Line 50:
	<pre>		<pre>
	SELECT * FROM product;		SELECT * FROM product;
		+	</pre>
		+
		+	== Restricting Number of Results ==
		+
		+	When retrieving data from a table it is possible to limnit the number of results that are returned by the ''SELECT'' statement using the ''LIMIT'' keyword. This keyword is followed by a number indicating how may rows are to be retrieved:
		+
		+	<pre>
		+	SELECT * FROM product LIMIT 10;
		+	</pre>
		+
		+	The above command will retrieve only 10 rows from the database table. Both the start and end rows can be specified for reading further into a table. The following example, therefore, extracts rows 10 through 15 of the table:
		+
		+	<pre>
		+	SELECT * FROM product LIMIT 10, 15;
	</pre>		</pre>

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Revision as of 18:52, 9 October 2007

Just a database system would be useless without some way to write data to the database files, a database would be similarly useless if there were no way to extract the stored data. Amongst the available SQL statements, one of the most frequently used is the SELECT statement. The purpose of the SELECT statement is to retrieve data from a database table based on specified criteria. In this chapter we will cover the use of the SELECT statement in detail.

Retrieving a Single Column

The most basic of SELECT statements simply retrieves a single column of all the rows of a table. The following SQL statements select a database named extract all the product_description column entries in the product table:

USE MySampleDB;
SELECT product_description FROM product;

Once executed, this command will display a list of every product description contained in the product table:

+-------------------+
| prod_desc         |
+-------------------+
| CD Writer         |
| Cordless Mouse    |
| SATA Disk Drive   |
| Ergonomic Keyboard|
+-------------------+
4 rows in set (0.00 sec)

Using SELECT to Retrieve Mutiple Columns

So far we have seen how easy it is to extract a single column from each row of a table. In the real world, it is more likely that information from more than one column will need to be retrieved. Fortunately the SELECT statement makes this task easy too. In fact, all that needs to be done is to specify the columns names after the SELECT statement, each separated by a comma. For example, to retrieve data from three columns in our database table:

SELECT prod_code, prod_name, prod_desc FROM product;

The above command will generate the following output if executed from within the mysql tool:

+-----------+--------------------------+-------------------+
| prod_code | prod_name                | prod_desc         |
+-----------+--------------------------+-------------------+
|         1 | CD-RW Model 4543         | CD Writer         |
|         2 | EasyTech Mouse 7632      | Cordless Mouse    |
|         3 | WildTech 250Gb 1700      | SATA Disk Drive   |
|         4 | Microsoft 10-20 Keyboard | Ergonomic Keyboard|
+-----------+--------------------------+-------------------+
4 rows in set (0.01 sec)

Whilst this approach works fine if you do not want to display all columns in a table, it can become cumbersome in situations where a table contains many columns and you want to list all columns. An easier way to achieve this than specifying every column is to use the wildcard symbol (*) in place of the column names. For example:

SELECT * FROM product;

Restricting Number of Results

When retrieving data from a table it is possible to limnit the number of results that are returned by the SELECT statement using the LIMIT keyword. This keyword is followed by a number indicating how may rows are to be retrieved:

SELECT * FROM product LIMIT 10;

The above command will retrieve only 10 rows from the database table. Both the start and end rows can be specified for reading further into a table. The following example, therefore, extracts rows 10 through 15 of the table:

SELECT * FROM product LIMIT 10, 15;