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Structured Query Language (SQL)

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A query is a more powerful type of filter that can gather information from multiple table in a relational database. For eg. The query can include field names such as "customer" and "city" from a customer table and "order date" from the order table as follows:

Some database systems provide social windows or forms for creating queries. Because of similar structure of almost all databases, a common type of query language is developed. In mid 1970's a language called Structured English Query Language (SEQUEL) was developed by mainframe database developers. Later SEQUEL was changed to SQL with some improvement in it. SEQUEL and SQL are the English like query language that allow user to query a database without much knowing about the underlying database.

Because SQL is such a powerful tool, most of the PC based DBMS have the SQL query tool but they can have some variation of the SQL with some extra feature added to it. Example of SQL implemented database systems are ORACLE, SYBASE, MYSQL, Microsoft Access etc.

In validation to SQL, PC based database sometime use a query programming language  called XBASE. like SQL, XBASE is somewhat English like, but it is more complicated because its command cover the full range of database activities not just queries.

QBE (Query by Example) is another form of query but it uses the graphical interface to query the database. with QBE, we specify the search criteria by typing values or expression into the fields of QBE form or grid.

Basic Structure
    SELECT field1, field2, .....
    FROM table name
    WHERE condition

Query from on table
    SELECT name, phoneno
    FROM stud_detail
    WHERE sec="A"

Query from two tables pub and title
    SELECT pub_name, title
    FORM pub, title
    WHERE pub.pubid = title.pubid


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