Structured Query Language
Structured Query Language (SQL) is a simple language designed for querying and managing Relational Database Management Systems (RDBMS). RDBMSs organize data using tables, where each table has named column(s). Each column has one datatype, and data is stored as rows intersecting with these columns and therefore satisfy the datatype of the corresponding column. For example, a student table could have two columns, student_id and student_name. If I want to insert data into this table, I would insert a row of two values, a student_id e.g. 001 and a student_name e.g. Claire. It is this concept of tables that is the basis of SQL, which has become the de facto standard adopted by most database management systems vendors. However, most vendors provide an extension to SQL to serve various functions ranging from the integration of procedural constructs to exception handling.
SQL Data Manipulation Language
SQL provides the ability to
- query databases to retrieve data
- insert records
- update records
- delete records
Syntax of the SELECT statement
The syntax of a simple SQL query:
SELECT select-list FROM table1, table2, ... WHERE predicates GROUP BY grouping-columns HAVING predicates
All rows in the table specified in the FROM clause are retrieved. If more than one table is specified, the Cartesian product of the rows is produced. Next, the rows not satisfying the predicates provided in the WHERE clause eliminated. Then the GROUP BY clause groups the remaining rows according to the equal values in the specified grouping-columns. Next, groups not satisfying the predicates specified in the HAVING clause are eliminated. Finally, the columns specified in the select-list are returned.
The WHERE,GROUP BY and HAVING clauses are optional.
Syntax of the INSERT statement
INSERT INTO table VALUES (value1, value2, ...)
Syntax of the UPDATE statement
UPDATE table SET column = value WHERE predicates
Syntax of the DELETE statement
DELETE FROM table WHERE predicates
SQL Data Definition Language
With the DDL, you can create and delete database tables. Furthermore, it allows the creation of other structures such as indexes and constraints.
Examples
Here is a simple example that returns details from a table called 'students', only if the row has a GPA higher than 3:
SELECT name, rollnum, totalmarks
FROM students
WHERE gpa > 3;
The columns 'name', 'rollnum' and 'totalmarks' must be present in the 'students' table, although the table could have other columns. The result is a set of rows that satisfy the condition (gpa > 3).
Queries can be nested in order to give more specific results as desired.