Educational

Category: Interview Question

  • Indexes (Clustered vs Non-Clustered)

      • Index → improves query speed (like book index).
      • Clustered Index → sorts and stores rows physically in table. (Only one allowed).
      • Non-Clustered Index → separate structure pointing to data. (Many allowed).

    1. View vs Table

      • Table → physical storage of data.
      • View → virtual table based on query.
        CREATE VIEW HighSalary AS
SELECT name, salary FROM Employees WHERE salary > 60000;

      1. UNION vs UNION ALL

          UNION → removes duplicates.

          UNION ALL → keeps duplicates.

        1. INNER JOIN vs LEFT JOIN

          • INNER JOIN → returns only matching rows.
          • LEFT JOIN → all rows from left + matched rows from right.
          • RIGHT JOIN → all rows from right + matched rows from left.
          • FULL JOIN → all rows from both sides.

        2. WHERE vs HAVING

          • WHERE → filters rows before grouping.
          • HAVING → filters groups after aggregation.
            SELECT department, COUNT(*) 
FROM Employees 
WHERE salary > 50000
GROUP BY department
HAVING COUNT(*) > 5;

          1. Primary Key Foreign Key

            Primary Key

            • Uniquely identifies each row.
            • A table can have only one primary key, but it may consist of multiple columns (composite key).

              Foreign Key

              • Ensures referential integrity by linking two tables.
                CREATE TABLE Orders (
   OrderID INT PRIMARY KEY,
   CustomerID INT,
   FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)
);

              1. DELETE vs TRUNCATE vs DROP

                  • DELETE → removes rows (can use WHERE).
                  • TRUNCATE → removes all rows (faster, cannot use WHERE).
                  • DROP → deletes the whole table structure.
                DELETE FROM Employees WHERE id=5;   -- removes one row
TRUNCATE TABLE Employees;           -- removes all rows
DROP TABLE Employees;               -- deletes table completely

              2. types of SQL commands?

                • DDL (Data Definition Language): CREATE, DROP, ALTER, TRUNCATE
                • DML (Data Manipulation Language): INSERT, UPDATE, DELETE
                • DQL (Data Query Language): SELECT
                • DCL (Data Control Language): GRANT, REVOKE
                • TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT

              3. What is SQL?

                SQL (Structured Query Language) is used to interact with relational databases (insert, update, delete, and query data).

                SQL is a database computer language designed for the retrieval and management of data in a relational database. SQL stands for Structured Query Language. This tutorial will give you a quick start to SQL. It covers most of the topics required for a basic understanding of SQL and to get a feel of how it works.