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Database query (Read 2452 times)
Gerrit-Jan Linker
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Posts: 75
Database query
05.12.08 at 21:08:52
 
Database query
 
With database query a search for data in the database is meant. The command language to interact with databases is SQL (Standard Query Language) and to retrieve information from a database a SQL select statement is used.  
 
Often there is much data stored in a database organised in several tables. The process to extract data from these tables is called querying, extracting, exporting. The SQL command that is used to query is the select statement. To proceed we need to get familiar with the SQL select statement.
 
SQL select statement
SQL commands are just simpel text commands that are sent to the database. E.g. when sending the command: select * from mytable the database will return all rows and all columns from the database table called mytable. This looks easy enough, doesn't it? Type another table name and you get the data from another table. Easy.
 
The where clause
Usually you don't want to download the whole table. You may for example only be interested in rows that have particular characteristics. For example in an exmployee table you may want to find all employees that earn more than 1000. This is where the where clause comes in.
 
The where clause is an addition to the select command we have seen so far. You simply type some conditions so the database returns only those rows that adhere to the conditions given. E.g.: select * from employees where sal > 1000
You can combine multiple conditions together. E.g.:
select * from employees
where sal > 1000
and not job = 'DIRECTOR'
and name like 'A%'
This would select rows of employeed that earn more than 1000, that are not a director and whose name start with an A.
 
Column list
You can be specific what data is returned by specifying the column list or to enter some formulas to process the data. For example if I am only interested to see the job title and the name I can write: select job, name from employees. Now only these two columns are returned.
 
Formulas can be used to do some processing or calculations on the data. Suppose there is a salary and a bonus column and you don't want to return them as separate columns but as a sum. You can write: select sal + bonus from employees.
 
Table joins
Sometimes the data to be retrieved is not in one table only but is stored in several tables. You can combine the data from multiple tables using joining statements in the where clause. Consider the situation that an employee works in a department. On the employee table the department id is stored and the departement name is stored in the departments table. You can retrieve all employee and department details running a query like this:
select * from employees, departments where employees.department_id = departments.id
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Gerrit-Jan Linker
Linker IT Software
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Gerrit-Jan Linker
YaBB Administrator
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Posts: 75
Re: Database query
Reply #1 - 08.01.09 at 09:57:12
 
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Gerrit-Jan Linker
Linker IT Software
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