Market99 Interview Questions and Answers

Union combines and removes duplicates, while union all combines without removing duplicates.

• Union removes duplicates from the result set, while union all does not.

• Union is slower than union all because it has to perform an additional step to remove duplicates.

• Union all is used when duplicates are desired in the result set.

• Example: SELECT column1 FROM table1 UNION SELECT column1 FROM table2;

• Example: SELECT column1 FROM table1 UNION ALL SELECT column1 FROM table2;

Primary key is a unique identifier for each record in a database table.

• Primary key ensures each record in a table is unique

• It can be a single column or a combination of columns

• Primary key values cannot be NULL

Union, Union All, and Difference are SQL set operators used to combine or compare data from multiple tables.

• Union combines the results of two or more SELECT statements and removes duplicates.

• Union All combines the results of two or more SELECT statements and includes duplicates.

• Difference (also known as Except) returns only the rows from the first SELECT statement that are not in the second SELECT statement.

• All three operators require that the SELECT statements have the same ...read more

Advanced SQL techniques for handling large datasets

• Use indexing to improve query performance

• Optimize queries by avoiding unnecessary joins and subqueries

• Utilize partitioning and clustering to manage large tables efficiently

• Consider using window functions for complex analytical queries

Slowly Changing Dimensions (SCD) include Type 1, Type 2, and Type 3 dimensions.

• Type 1: Overwrite existing data with new data, no history is kept.

• Type 2: Create a new record for each change, maintaining history.

• Type 3: Create a new attribute to store changes, keeping limited history.