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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.
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 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) |
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) |
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) |
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