SQL Tables

A SQL table is a structured collection of data, systematically organized into rows and columns. In the realm of database management systems (DBMS), a table is referred to as a relation, while rows are termed as tuples.

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Key Features of SQL Tables

1. Fixed Columns, Variable Rows:
   A table has a predefined number of columns, but it can accommodate any number of rows. This design makes tables a straightforward and efficient way to store data.
2. Simple Representation of Relations:
   Tables are not just about storing data; they represent relationships among data points, making them invaluable in relational databases.

Example: Employee Table

Consider the following example:

EMP_NAME	ADDRESS	SALARY
Ajay	Arunachal Pradesh	15000
Aman	Bihar(Patna)	18000
Rohit	Arkansas	20000

• Table Name: Employee
• Column Names: EMP_NAME, ADDRESS, and SALARY
• Rows (Tuples): Each combination of data from the columns forms a row. For instance, Ankit, Lucknow, and 15000 constitute one row.

SQL Table Variables

A table variable in SQL is a special type of variable that stores data temporarily. Introduced by Microsoft in SQL Server 2000, table variables serve as a lightweight alternative to temporary tables.

Features of SQL Table Variables

1. Temporary Storage:
   Table variables temporarily store records and results, functioning similarly to temporary tables.
2. Resource Efficiency:
   Table variables consume fewer resources compared to temporary tables.
3. No Explicit Drop Required:
   Unlike temporary tables, table variables do not need explicit deletion as they are automatically managed.
4. Data Persistence During Transactions:
   Data in table variables is not rolled back when a transaction is rolled back.
5. Parameter Restrictions:
   Table variables cannot be used as input or output parameters.

Syntax for Creating a Table

The syntax for creating a table is straightforward and resembles the declaration of table variables:

CREATE TABLE table_name (  
    column1 data_type,  
    column2 data_type,  
    ...  
    columnN data_type  
);  

SQL TABLE Statements Overview

1. SQL CREATE TABLE

• Used to define a new table in the database.
• Example: CREATE TABLE Employee ( EMP_NAME VARCHAR(50), ADDRESS VARCHAR(100), SALARY DECIMAL(10, 2) );

2. SQL DROP TABLE

• Deletes an existing table and all its data.
• Example: DROP TABLE Employee;

3. SQL DELETE TABLE

• Removes all records from a table while retaining its structure.
• Example: DELETE FROM Employee;

4. SQL RENAME TABLE

• Changes the name of a table.
• Example: EXEC sp_rename 'Employee', 'Staff';

5. SQL TRUNCATE TABLE

• Quickly deletes all rows from a table without logging individual row deletions.
• Example: TRUNCATE TABLE Employee;

6. SQL COPY TABLE

• Creates a duplicate table with the same structure and/or data.
• Example: SELECT * INTO EmployeeCopy FROM Employee;

7. SQL TEMP TABLE

• Temporary tables exist for the duration of a session or procedure.
• Example: CREATE TABLE #TempEmployee ( EMP_NAME VARCHAR(50), ADDRESS VARCHAR(100), SALARY DECIMAL(10, 2) );

8. SQL ALTER TABLE

• Modifies the structure of an existing table (e.g., adding, renaming, or dropping columns).
• Example: ALTER TABLE Employee ADD Email VARCHAR(100);

Advantages of Temporary Tables

• Session Isolation: Temporary tables are session-specific and get automatically dropped after the session ends.
• Data Isolation: They prevent conflicts in multi-user environments.

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