Which MySQL command helps to add a primary key constraint to any table that has already been created ? (A) UPD

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Q. Which MySQL command helps to add a primary key constraint to any table that has already been created ? (A) UPDATE (B) INSERT INTO (C) ALTER TABLE (D) ORDER BY

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Concept understanding — SQL Query Concepts

SQL Query Concepts — A First Look

Think of a library. You walk in, and instead of wandering through endless shelves, you go to the librarian and say: "Give me all the books by Tagore published after 1950, sorted by title." The librarian knows exactly where everything is, finds what you asked for, and hands you the list. That's what SQL does — but for data stored in a computer.

SQL stands for Structured Query Language. It is the language we use to talk to databases. A database is just a collection of related information stored in tables — like an Excel workbook with multiple sheets, but far more powerful and organised.

The core idea: tables and queries

A database stores data in tables. Each table has rows (records) and columns (fields). For example, a Students table might have columns like RollNo, Name, City, and Marks. Each row is one student's complete information.

A query is simply a question you ask the database. You write it in SQL, and the database returns an answer — usually a new table made from the data that matches your question.

Note

SQL is not a programming language like Python or Java. You don't tell the computer how to find the data. You only say what data you want. The database figures out the fastest way to get it.

The most common SQL command: SELECT

Almost every query begins with SELECT. It means "show me this data." The basic structure is:

SELECT column1, column2 FROM table_name WHERE condition;

Read it like a sentence: "Select these columns from this table where this condition is true."

If you want all columns, you write SELECT *. The asterisk is a shortcut meaning "everything."

Filtering with WHERE

The WHERE clause is your filter. It keeps only the rows that satisfy a condition. For example, if you want students from Delhi only, you write:

SELECT * FROM Students WHERE City = 'Delhi';

You can combine conditions using AND (both must be true) and OR (at least one must be true). This is exactly like everyday logic: "Show me students from Delhi AND who scored above 80."

Sorting with ORDER BY

Data often comes back in no particular order. To arrange it, use ORDER BY. You can sort ascending (smallest first, which is the default) or descending (largest first) using DESC.

SELECT Name, Marks FROM Students ORDER BY Marks DESC;

This gives you a ranked list — highest marks first.

Why SQL matters for commerce and humanities

You will never calculate a formula in SQL. That is not what it does. What SQL does is organise, filter, and retrieve information — and that is the foundation of every data-driven decision. …

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Concept: DDL commands for modifying table structure after creation.

When a table already exists and you need to add constraints like a primary key, you must use a Data Definition Language (DDL) command that modifies the table's schema. The ALTER TABLE command is specifically designed for this purpose—it allows you to add, drop, or modify columns and constraints on existing tables.

The syntax to add a primary key constraint is:

ALTER TABLE table_name ADD PRIMARY KEY (column_name);
``` …

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Primary key constraints on existing tables are added through schema modification commands. The answer is (C) ALTER TABLE.

When you create a table in MySQL, you define its structure: column names, data types, and constraints like primary keys. But what happens when you've already created a table and later realize you need to add a primary key? You need a command that modifies the table's structure itself, not its data.

This is where the distinction between Data Definition Language (DDL) and Data Manipulation Language (DML) becomes crucial. DDL commands change the schema—the blueprint of your database objects. DML commands work with the data inside those objects.

Let's examine each option:

UPDATE is a DML command that modifies existing data in rows. You use it to change values in columns, like UPDATE students SET grade = 'A' WHERE id = 5. It cannot alter the table's structure or add constraints.
INSERT INTO is another DML command that adds new rows of data to a table. It populates the table with records but has no capability to modify the table definition itself.
ALTER TABLE is the DDL command specifically designed to modify an existing table's structure. You can add columns, drop columns, change data types, and crucially, add or remove constraints like primary keys, foreign keys, and unique constraints. …

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Q.Which MySQL command helps to add a primary key constraint to any table that has already been created ? (A) UPDATE (B) INSERT INTO (C) ALTER TABLE (D) ORDER BY

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