1. What is SQL?
SQL (Structured Query Language) is a standard programming language used to manage and manipulate databases. It is used for Creating, inserting, updating, and deleting data in relational database management systems (RDBMS).
2. What are the different types of joins in SQL?
INNER JOIN: Returns records that have matching values in both tables.
LEFT JOIN (or LEFT OUTER JOIN): Returns all records from the left table and the matched records from the right table.
RIGHT JOIN (or RIGHT OUTER JOIN): Returns all records from the right table and the matched records from the left table.
FULL JOIN (or FULL OUTER JOIN): Returns records when there is a match in either left or right table.
CROSS JOIN: Returns the Cartesian product of the two tables.
3. What is the difference between WHERE and HAVING clauses?
WHERE is used to filter records before any grouping takes place.
HAVING is used to filter records after the GROUP BY operation.
4. What is a Primary Key?
A primary key is a unique identifier for a record in a table. It ensures that no two rows have the same values in the primary key column(s) and that NULL values are not allowed.
5. What is a Foreign Key?
A foreign key is a column or a combination of columns used to establish a link between the data in two tables. It references the primary key of another table to maintain referential integrity.
6. What is normalization?
Normalization is the process of organizing the fields and tables of a database to minimize redundancy and dependency. It involves dividing large tables into smaller ones and defining relationships between them.
7. What is a JOIN? Can you explain the different types of joins?
A JOIN is a SQL operation used to combine rows from two or more tables based on a related column. The types of joins include INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN, and CROSS JOIN.
8. What are Aggregate Functions in SQL?
Aggregate functions are used to perform calculations on multiple values and return a single result. Common aggregate functions are:
COUNT(): Returns the number of rows.
SUM(): Returns the sum of values.
AVG(): Returns the average of values.
MAX(): Returns the maximum value.
MIN(): Returns the minimum value.
9. What is the difference between UNION and UNION ALL?
UNION: Combines the result sets of two or more queries, removing duplicates.
UNION ALL: Combines the result sets of two or more queries, including duplicates.
10. What is the NULL value in SQL?
A NULL value represents the absence of a value or an unknown value. It is different from an empty string or zero and is used to indicate that a data field has no value.
