Q111. An unique (non-key) field?
To enforce that every value in a non-primary key column is unique, apply the **`UNIQUE`** constraint: ```sql CREATE TABLE users ( id INT PRIMARY KEY, email VARCHAR(255) U...
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Q112. What is CHECK constraint?
A **`CHECK`** constraint ensures that all values stored in a column satisfy a specific boolean condition: ```sql CREATE TABLE products ( product_id INT PRIMARY KEY, name ...
Q113. UNIQUE vs PRIMARY KEY?
| PRIMARY KEY | UNIQUE | |-------------|--------| | One per table | Multiple per table | | Cannot be NULL | Usually allows one NULL | | Implicitly indexed | Explicitly in...
Q114. What does DROP TABLE do?
Permanently removes the table definition, all data, indexes, triggers, and constraints. **Cannot be undone** (unless in a transaction that rolls back, in some DBs). ```sq...
Q115. DROP vs TRUNCATE?
While both `DROP` and `TRUNCATE` are DDL operations, their scope is fundamentally different: | Feature | `TRUNCATE TABLE` | `DROP TABLE` | | :--- | :--- | :--- | | **Data...
Q116. Add order_date column to order table?
To add an `order_date` column to an existing table, use `ALTER TABLE ... ADD COLUMN`: ```sql ALTER TABLE `orders` ADD COLUMN order_date DATE NOT NULL DEFAULT (CURRENT_DAT...
Q117. Rename column Address to Addr in Customer?
```sql -- MySQL ALTER TABLE Customer CHANGE Address Addr VARCHAR(50); -- PostgreSQL / Standard ALTER TABLE Customer RENAME COLUMN Address TO Addr; ```
Q118. Delete column CITY from ADDRESSES?
To remove a column from an existing table schema, use `ALTER TABLE ... DROP COLUMN`: ```sql ALTER TABLE addresses DROP COLUMN city; ``` *Production Warning:* Dropping a c...
Q119. What are Indexes?
Data structures (typically B-Trees) that speed up data retrieval at the cost of slower writes and extra storage. Think of them as a book's table of contents.
Q120. Clustered vs Non-Clustered?
- **Clustered**: Determines physical storage order. One per table. The data IS the index. - **Non-Clustered**: Separate structure with pointers to data. Multiple allowed ...