Q81. DELETE vs TRUNCATE?
See the comparison table above. Key differences: `DELETE` is DML (row-by-row, rollbackable, fires triggers, has WHERE). `TRUNCATE` is DDL (bulk page deallocation, faster,...
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Q82. Which statement creates a table?
The **`CREATE TABLE`** statement defines and provisions a new table schema: ```sql CREATE TABLE Customers ( customer_id INT AUTO_INCREMENT PRIMARY KEY, first_name VARCHAR...
Q83. What is database collation and how does it affect SQL queries?
Collation defines the **rules for comparing and sorting** characters. It works with the character set. - `utf8mb4_general_ci`: Case-insensitive, faster - `utf8mb4_unicode...
Q84. What is AUTO_INCREMENT?
Automatically generates sequential unique integers for a column, typically the PRIMARY KEY. ```sql CREATE TABLE users (id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(50)...
Q85. Valid constraints in MySQL?
MySQL supports a complete set of integrity constraints: 1. **`PRIMARY KEY`**: Uniquely identifies each row (implies `NOT NULL` and `UNIQUE`). 2. **`FOREIGN KEY`**: Enforc...
Q86. Which is NOT TRUE about constraints?
Any assertion claiming that **constraints are optional in production** or that **constraints significantly slow down reads** is false. ### Why Constraints Are Vital: - Co...
Q87. NOT NULL constraint?
The **`NOT NULL`** constraint enforces that a column must always contain a valid data value and cannot accept `NULL`: ```sql CREATE TABLE employees ( id INT PRIMARY KEY, ...
Q88. 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...
Q89. 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 ...
Q90. UNIQUE vs PRIMARY KEY?
| PRIMARY KEY | UNIQUE | |-------------|--------| | One per table | Multiple per table | | Cannot be NULL | Usually allows one NULL | | Implicitly indexed | Explicitly in...