DDL (CREATE, ALTER, DROP) in DBMS Theory - Step-by-Step Execution

Concept Flow - DDL (CREATE, ALTER, DROP)

Start

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Choose DDL Command

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CREATE

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Define

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New Object

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Execute Command

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Object Structure Updated

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End

The flow starts by choosing a DDL command: CREATE to make new objects, ALTER to change existing ones, or DROP to remove them, then executes and updates the database structure.

Execution Sample

DBMS Theory

CREATE TABLE Students (ID INT, Name VARCHAR(50));
ALTER TABLE Students ADD COLUMN Age INT;
DROP TABLE Students;

This sequence creates a table, adds a column, then deletes the table.

Analysis Table

Step	Command	Action	Result
1	CREATE TABLE Students (ID INT, Name VARCHAR(50));	Create new table named Students with columns ID and Name	Table Students created with 2 columns
2	ALTER TABLE Students ADD COLUMN Age INT;	Add new column Age to Students table	Students table now has columns ID, Name, Age
3	DROP TABLE Students;	Remove the Students table from database	Students table deleted
4	-	No more commands	Execution ends

💡 All commands executed; database structure updated accordingly.

State Tracker

Database Object	Start	After Step 1	After Step 2	After Step 3
Students Table	Does not exist	Exists with columns: ID, Name	Exists with columns: ID, Name, Age	Does not exist

Key Insights - 3 Insights

Why does the Students table not exist after the DROP command?

Because the DROP command removes the entire table from the database, as shown in execution_table step 3 where the table is deleted.

Can ALTER add a column to a table that does not exist?

No, ALTER only works on existing tables. In the execution_table, step 2 shows ALTER adding a column after the table was created in step 1.

Does CREATE overwrite an existing table?

No, CREATE fails if the table already exists unless specified otherwise. Here, the table is created first, so no overwrite occurs.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what change happens to the Students table?

A The table is renamed

B A new column Age is added

C The table is deleted

D A column is removed

💡 Hint

Check the 'Action' and 'Result' columns in execution_table row 2.

At which step does the Students table stop existing?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Result' column in execution_table step 3.

If the DROP command was omitted, what would be the final state of the Students table?

A Table would have columns ID, Name, and Age

B Table would have columns ID and Name only

C Table would not exist

D Table would be renamed

💡 Hint

Refer to variable_tracker after step 2 showing columns present.

Concept Snapshot

DDL commands change database structure:
- CREATE makes new objects (tables, etc.)
- ALTER changes existing objects (add/remove columns)
- DROP deletes objects
Use these to manage database schema safely.

Full Transcript

This visual execution shows how DDL commands CREATE, ALTER, and DROP work step-by-step. First, CREATE makes a new table called Students with two columns. Then ALTER adds a new column Age to that table. Finally, DROP removes the Students table entirely. The execution table tracks each command's action and result, while the variable tracker shows the table's state after each step. Key moments clarify that DROP deletes the table, ALTER only works on existing tables, and CREATE does not overwrite existing tables. The quiz tests understanding of these changes by referencing the execution steps and variable states.