SQL Expressions

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 (.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)


SQL-CREATE DATABASE:

The SQL CREATE DATABASE statement is used to create new SQL database.

Syntax:

Basic syntax of CREATE DATABASE statement is as follows:

CREATE DATABASE DatabaseName;

Always database name should be unique within the RDBMS.
Example:

If you want to create new database <testDB>, then CREATE DATABASE statement would be as follows:

SQL> CREATE DATABASE testDB;

Make sure you have admin privilege before creating any database. Once a database is created, you can check it in the list of databases as follows:

SQL> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| AMROOD             |
| TUTORIALSPOINT     |
| mysql              |
| orig               |
| test               |
| testDB             |
+--------------------+
7 rows in set (0.00 sec)


SQL-DROP DATABASE:

The SQL DROP DATABASE statement is used to drop an existing database in SQL schema.

Syntax:

Basic syntax of DROP DATABASE statement is as follows:

DROP DATABASE DatabaseName;

Always database name should be unique within the RDBMS.
Example:

If you want to delete an existing database <testDB>, then DROP DATABASE statement would be as follows:

SQL> DROP DATABASE testDB;

NOTE: Be careful before using this operation because by deleting an existing database would result in loss of complete information stored in the database.

Make sure you have admin privilege before dropping any database. Once a database is dropped, you can check it in the list of databases as follows:

SQL> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| AMROOD             |
| TUTORIALSPOINT     |
| mysql              |
| orig               |
| test               |
+--------------------+
6 rows in set (0.00 sec)

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