Q.Write the full form of the following abbreviations:

The concept: degree of a relation

In relational database terminology, the degree of a relation (table) is simply the number of attributes (columns) it contains. For example, a Student table with columns RollNo, Name, Age has degree 3.

If you want to change the degree, you must either add a new column (increasing the degree) or remove an existing column (decreasing the degree). This is a structural change to the table schema — not a change to the data inside the table.

Why each option does or doesn't change the degree

Let's examine each SQL command:

1. DROP TABLE — This removes the entire table from the database. It doesn't change the degree of an existing relation; it destroys the relation itself. So this is not about altering degree.

2. ALTER TABLE — This command is specifically designed to modify the structure of an existing table. You can use it with clauses like:

   • ADD column_name datatype — adds a new column, increasing the degree.
   • DROP COLUMN column_name — removes an existing column, decreasing the degree.

   Therefore, ALTER TABLE can directly change the degree of a relation.

3. UPDATE…SET — This modifies the values of existing rows in a table. It changes the content of cells, but it never adds or removes columns. The degree remains exactly the same.

4. DELETE — This removes rows (tuples) from a table. It changes the cardinality (number of rows), but not the degree. The column structure is untouched. …

The key to this question is understanding the four categories of SQL commands: DDL, DML, DCL, and TCL. Most exam questions test whether you can tell apart DDL (structure-changing) from DML (data-changing).

DDL (Data Definition Language) commands define or modify the schema — the structure of tables, databases, indexes, etc. They include CREATE, ALTER, DROP, TRUNCATE, RENAME. These commands auto-commit and cannot be rolled back in most databases.

DML (Data Manipulation Language) commands work on the data inside existing tables — inserting, updating, deleting, or retrieving rows. The classic DML commands are SELECT, INSERT, UPDATE, DELETE. Some exams also include MERGE (upsert) and CALL (for procedures) as DML, but the core four are what matter here.

Now let's check each option:

1. Option (A): UPDATE — This modifies existing rows in a table. It changes data values, not the table structure. This is a classic DML command. ✓ …

Understanding DDL vs DML in SQL

SQL commands fall into distinct categories based on their purpose. Data Definition Language (DDL) commands deal with the structure and schema of database objects—tables, indexes, views, and so on. They define what exists in the database, not the data itself.

When you create a table, drop it, or alter its columns, you're changing the blueprint. These structural operations are DDL. In contrast, Data Manipulation Language (DML) commands work with the actual data inside those structures—inserting rows, updating values, deleting records, or querying them.

The distinction matters because DDL commands typically auto-commit (changes are permanent immediately) and affect metadata, while DML commands operate on data and can be rolled back within transactions.

Examining each command

1. DROP – Removes an entire database object (table, view, index) from the schema. This is pure structure manipulation: DROP TABLE employees; deletes the table definition itself, along with all its data. Clearly DDL.

2. CREATE – Defines a new database object. Whether CREATE TABLE, CREATE INDEX, or CREATE VIEW, you're building structure. The quintessential DDL command. …

The question asks about the specific SQL statement used to add new records into a table. To understand this, we need to recall the fundamental categories of SQL commands: Data Definition Language (DDL) and Data Manipulation Language (DML).

DDL commands are used to define and manage the database schema or structure. Think of them as commands that build or modify the "blueprint" of your database, like creating tables, altering their structure, or dropping them entirely.

DML commands, on the other hand, are used to manage the data within those defined structures. These are the commands you use to interact with the actual information stored in your tables—adding new data, changing existing data, or removing data.

The task of adding new records clearly falls under manipulating the data, not defining the structure. Therefore, we are looking for a DML command.

1. Understand the Goal: The objective is to add new rows or records to an existing table. This means we are dealing with the content of the table, not its structure.

2. Evaluate Option (a) ALTER:

   The ALTER statement is a DDL command. It is used to modify the structure of an existing database object, such as adding a new column to a table, dropping an existing column, or changing the data type of a column. It does not insert new data rows.

   For example: ALTER TABLE Employees ADD COLUMN Email VARCHAR(100);

3. Evaluate Option (b) UPDATE:

   The UPDATE statement is a DML command. It is used to modify existing records in a table. If you want to change the value in one or more columns for rows that already exist, you use UPDATE. It does not create new rows.

   For example: UPDATE Employees SET Salary = 60000 WHERE EmployeeID = 101;

   Watch out

   A common mistake is to confuse UPDATE with INSERT. Remember, UPDATE changes data in existing rows, while INSERT adds new rows.

4. Evaluate Option (c) INSERT: …

Why ALTER TABLE?

When you create a table using CREATE TABLE, you define its initial structure—the columns, their data types, constraints, and so on. But database schemas evolve. You might realize later that you need to store additional information: perhaps a phone number column in a customer table, or a timestamp for when records were last modified.

SQL provides the ALTER TABLE command specifically for modifying the structure of existing tables. Think of it as the "edit mode" for your table's blueprint. Among its various capabilities—renaming columns, dropping columns, changing data types—the ADD clause lets you introduce new attributes without losing any existing data.

The ADD Clause in Action

The basic syntax is straightforward:

ALTER TABLE table_name
ADD column_name datatype [constraints];

Here's what happens step by step:

1. Identify the table: ALTER TABLE targets the specific table you want to modify.

2. Specify the operation: The ADD keyword tells the database you're introducing a new column, not removing or changing an existing one.

3. Define the new attribute: You provide the column name, its data type (like VARCHAR(50), INT, DATE), and optionally any constraints (NOT NULL, DEFAULT value, etc.).

4. Execution: The database adds the column to every row. For existing rows, the new column is populated with NULL (unless you specify a DEFAULT value).

Example …

Database management systems organize commands into distinct categories based on what they do. Understanding these categories helps you grasp how databases are structured versus how data within them is actually used.

Data Definition Language (DDL) handles the skeleton of your database. It defines the structure—creating tables, altering their schema, setting up constraints, and establishing relationships. When you need to build or modify the architecture itself, you're working with DDL. Common DDL commands include CREATE, ALTER, DROP, and TRUNCATE.

Data Manipulation Language (DML), on the other hand, works with the actual data inside those structures. Once your tables exist, DML lets you insert records, update values, delete entries, and retrieve information. The classic DML commands are SELECT, INSERT, UPDATE, and DELETE. …