SQL is a database language that facilitates querying for and storing data in a relational database. Originally, SQL was known as SEQUEL (Structured English QUEry Language), which was introduced in 1974. In 1979, it became known as SEQUEL/2 when the American National Standards Institute approved it as an American Standard. In 1989, the name was changed to Structured Query Language - this eventually became SQL Server once Microsoft released their own relational database management system version of SQL in 1992 with SQL Server V6. SQL is a directed acyclic graph (DAG) and is the most widely used database language. It is also considered an important technology for modern business applications. In SQL, a query is a statement which states what data to return or what data to examine. A query can be executed using any of the following: In some cases however, these operations are not enough to get the required results so additional functions from either ANSI-92 from Extended SQL or ANSI-93 from Standard SQL are used. An example of a statement that cannot be translated into Standard SQL would be: This just returns all "products" that have no "manufacturers". In Standard SQL, this statement could be translated as: Using a CASE or a FOR clause, a query can be constructed to return values from different columns. Using a CASE FOR clause, the output of the first query can be used as input for subsequent queries. In Standard SQL, this is done as: In Standard SQL, an alternative syntax that builds on CASE could also be used as: In both cases above, the output from the first query would have been used as input for subsequent queries. This type of statement is called an IF-ELSE or "isolation" statement. SQL has four types of statements: Data Manipulation Language (DML), Data Definition Language (DDL), Connection Management (CMD) and Transaction Management (TM). DML statements are used to retrieve or insert data into a database. DDL statements are used to create or modify the structure of the database itself. CMD statements are used to control the connection between SQL and an outside application while TM statements ensure that all data is properly saved in the database before termination. Data Manipulation Language or DML consists of five basic commands which include SELECT, INSERT, UPDATE, DELETE and SQLCODE.
The SELECT statement is most commonly used as most queries involve retrieving information from a database. If a complex query is needed, the following criteria may be constructed: In the case of the example above, there are five different ways to construct a complex query. In general, any DML statement can be used anywhere in the WHERE clause while using an aggregate function. Functions like MAX() and MIN() can be used in any part of the statement while other functions must be placed at the end to ensure that only one function is used per column. An aggregate function should not be placed in front of other functions nor should other functions precede an aggregate function. SQLCODE allows control to exit from a SQL script while it's still running without terminating SQL Server itself.
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