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SQL is a must-have skill for database management, and interviewers love testing it! Whether you’re a beginner or prepping for your first SQL job, these common interview questions will help you ace your SQL interview.
You Should Know:
Here are some practical SQL commands and steps to help you practice and master SQL concepts:
1. Basic SQL Commands:
– `SELECT * FROM table_name;` – Retrieves all data from a table.
– `INSERT INTO table_name (column1, column2) VALUES (value1, value2);` – Inserts data into a table.
– `UPDATE table_name SET column1 = value1 WHERE condition;` – Updates existing data.
– `DELETE FROM table_name WHERE condition;` – Deletes data from a table.
2. Filtering and Sorting:
– `SELECT * FROM table_name WHERE condition;` – Filters data based on a condition.
– `SELECT * FROM table_name ORDER BY column_name ASC/DESC;` – Sorts data in ascending or descending order.
3. Joins:
– `SELECT * FROM table1 INNER JOIN table2 ON table1.column = table2.column;` – Combines rows from two tables based on a related column.
– `SELECT * FROM table1 LEFT JOIN table2 ON table1.column = table2.column;` – Returns all rows from the left table and matched rows from the right table.
4. Aggregate Functions:
– `SELECT COUNT(column_name) FROM table_name;` – Counts the number of rows.
– `SELECT AVG(column_name) FROM table_name;` – Calculates the average value.
– `SELECT SUM(column_name) FROM table_name;` – Calculates the sum of values.
5. Grouping Data:
– `SELECT column_name, COUNT(*) FROM table_name GROUP BY column_name;` – Groups rows that have the same values into summary rows.
6. Subqueries:
– `SELECT * FROM table_name WHERE column_name = (SELECT column_name FROM table_name WHERE condition);` – Nested query to perform complex operations.
7. Creating and Modifying Tables:
– `CREATE TABLE table_name (column1 datatype, column2 datatype);` – Creates a new table.
– `ALTER TABLE table_name ADD column_name datatype;` – Adds a new column to an existing table.
– `DROP TABLE table_name;` – Deletes a table.
8. Indexing:
– `CREATE INDEX index_name ON table_name (column_name);` – Creates an index on a table column for faster search.
9. Transactions:
– `BEGIN TRANSACTION;` – Starts a transaction.
– `COMMIT;` – Saves changes made during the transaction.
– `ROLLBACK;` – Reverts changes made during the transaction.
10. Backup and Restore:
- Use `mysqldump` for MySQL to backup a database:
mysqldump -u username -p database_name > backup_file.sql
- Restore a database:
mysql -u username -p database_name < backup_file.sql
What Undercode Say:
Mastering SQL is essential for anyone working with databases, whether you’re a developer, data analyst, or database administrator. The commands and steps provided above will help you practice and solidify your understanding of SQL. For further learning, consider exploring online courses or tutorials on platforms like W3Schools or SQLZoo. Keep practicing, and you’ll be well-prepared to tackle any SQL interview!
References:
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