An expression is a combination of one or more values, operators and SQL functions that evaluate to a value. These SQL EXPRESSIONs are like formulae and they are written in query language. You can also use them to query the database for a 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 −
- Boolean
- Numeric
- Date
Let us now discuss each of these in detail.
Boolean Expressions
SQL Boolean Expressions fetch the data based on matching a single value. Following is the syntax −
SELECT column1, column2, columnN FROM table_name WHERE SINGLE VALUE MATCHING 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)
The following table is a simple example showing the usage of various 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)
Numeric Expression
These expressions are 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, the numerical_expression is used for a mathematical expression or any formula. Following is a simple example showing the 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 the 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)
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 shown below −
SQL> SELECT GETDATE();; +-------------------------+ | GETDATE | +-------------------------+ | 2009-10-22 12:07:18.140 | +-------------------------+ 1 row in set (0.00 sec)
