SQL - A Set Based Declarative Query Language

SQL is a computer database language which is often referred as Structure Query Language. It is designed to maintain database in RDMS (relational database management system). It is originally based on relational algebra. Its scope includes data access control, modification, schema creation, and query, update, delete and data insertion.
Criticism the Language Face
 

For relational database it is most widely used language. In the early 1970s it was developed by Donald D. Chamberlin and Raymond F. Boyce at IBM. The common criticism which this language face includes complex along with ambiguous language of grammar and semantics, there is an inappropriate handling of missing data and there is lack of cross platform portability between vendors.
Clauses, Expressions, Predicate, Queries and Statements
 

The SQL is further divided into various language elements such as, Clauses - these are constituent components of queries and statements, Expressions - these gives scalar values or tables which contain rows and columns of data, Predicate - to change the program flow these are used to limit the effect of queries and statement, Queries - based on specific criteria they retrieves data, Statements - they have persistent effect on data along with schema and control diagnostics, sessions, connections, program flow and transactions.
 

Semicolon which is used in SQL statement as a statement terminator is defined as a standard part of the SQL grammar. In SQL queries and statements insignificant white spaces are ignored, this make convenient to format SQL code for comfortable readability.
Clauses in SQL - FORM, JOIN, WHERE, GROUP BY, ORDER BY
Query is the common operation in SQL and is executed by declarative SELECT statement. From one or more table or expressions, the SELECT statement retrieves data. To specify that the query should return all columns of the queried tables an asterisk ("*") is used.
 

In SQL the most complex statement is SELECT with various clauses and keywords such as, FORM clause - points the table from which the data is retrieved, it contain JOIN sub clause which specifies rules for joining the tables, WHERE clause - it consist of comparison predicate, GROUP BY clause - a smaller set of rows is used to project rows having common values, HAVING clause - it is used to filter clause which are resulted from the GROUP BY clause, ORDER BY clause - it identifies columns which are used to sort the resulting data.
 

DML - A Subset of SQL
Next comes the DML (Data Manipulation Language), it is the subset of SQL and is used to add, update and delete data. SQL is not an imperative language such as C or BASIC, but it is a set based, declarative query language.

 

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