SQL - Expressions

An expression is a combination of one or more values, operators, and SQL functions that evaluate to a value.
SQL EXPRESSIONs are like formulas and they are written in query language. You can also used to query the database for specific set of data.

Syntax:

Consider the basic syntax of the SELECT statement as follows:

SELECT column1, column2, columnN FROM table_name WHERE [CONTION|EXPRESSION];

There are different types of SQL expression, which are mentioned below:

SQL - Boolean Expressions:

SQL Boolean Expressions fetch the data on the basis of matching single value. Following is the syntax:

SELECT column1, column2, columnN FROM table_name WHERE SINGLE VALUE MATCHTING EXPRESSION;

Consider CUSTOMERS table has following records:

SQL> SELECT * FROM CUSTOMERS; +----+----------+-----+-----------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+----------+-----+-----------+----------+ | 1 | Ramesh | 32 | Ahmedabad | 2000.00 | | 2 | Khilan | 25 | Delhi | 1500.00 | | 3 | kaushik | 23 | Kota | 2000.00 | | 4 | Chaitali | 25 | Mumbai | 6500.00 | | 5 | Hardik | 27 | Bhopal | 8500.00 | | 6 | Komal | 22 | MP | 4500.00 | | 7 | Muffy | 24 | Indore | 10000.00 | +----+----------+-----+-----------+----------+ 7 rows in set (0.00 sec)

Here is simple examples showing usage of SQL Boolean Expressions:

SQL> SELECT * FROM CUSTOMERS WHERE SALARY = 10000; +----+-------+-----+---------+----------+ | ID | NAME | AGE | ADDRESS | SALARY | +----+-------+-----+---------+----------+ | 7 | Muffy | 24 | Indore | 10000.00 | +----+-------+-----+---------+----------+ 1 row in set (0.00 sec)

SQL - Numeric Expression:

This expression is used to perform any mathematical operation in any query. Following is the syntax:

SELECT numerical_expression as OPERATION_NAME [FROM table_name WHERE CONDITION] ;

Here numerical_expression is used for mathematical expression or any formula. Following is a simple examples showing usage of SQL Numeric Expressions:

SQL> SELECT (15 + 6) AS ADDITION +----------+ | ADDITION | +----------+ | 21 | +----------+ 1 row in set (0.00 sec)

There are several built-in functions like avg(), sum(), count() etc.to perform what is known as aggregate data calculations against a table or a specific table column.

SQL> SELECT COUNT(*) AS "RECORDS" FROM CUSTOMERS; +---------+ | RECORDS | +---------+ | 7 | +---------+ 1 row in set (0.00 sec)

SQL - Date Expressions:

Date Expressions return current system date and time values:

SQL> SELECT CURRENT_TIMESTAMP; +---------------------+ | Current_Timestamp | +---------------------+ | 2009-11-12 06:40:23 | +---------------------+ 1 row in set (0.00 sec)

Another date expression is as follows:

SQL> SELECT GETDATE();; +-------------------------+ | GETDATE | +-------------------------+ | 2009-10-22 12:07:18.140 | +-------------------------+ 1 row in set (0.00 sec)