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 use them 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 [CONDITION|EXPRESSION];

There are different types of SQL expressions, 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 the CUSTOMERS table having the 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 example 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)