SQL Mastery

What is the purpose of the SQL SELECT statement?

• To delete data from a database

• To retrieve data from a database.

• To update data in a database

• To create a new database

Explain the difference between INNER JOIN and OUTER JOIN in SQL.

• INNER JOIN returns all the rows from at least one of the tables

• OUTER JOIN returns only the matching rows from both tables

• INNER JOIN fills unmatched rows with NULL values

• INNER JOIN returns only the matching rows from both tables, while OUTER JOIN returns all the rows from at least one of the tables, with the unmatched rows filled with NULL values.

What are the SQL aggregation functions and how are they used?

• SQL aggregation functions are used to create new tables in a database

• SQL aggregation functions are used to update multiple records in a table

• SQL aggregation functions are used to perform calculations on a set of values and return a single value.

• SQL aggregation functions are used to delete data from a table

How can subqueries be used in SQL?

• By retrieving data from one table and using that result in another query as a condition or filter.

• By inserting data from one table and using that result in another query

• By deleting data from one table and using that result in another query

• By updating data from one table and using that result in another query

What are indexes in SQL and how do they improve performance?

• Indexes in SQL are only used for sorting data in a database table

• Indexes in SQL are irrelevant and have no impact on performance

• Indexes in SQL are data structures that improve the speed of data retrieval operations on a database table. They do this by providing quick access to the rows in the table, based on the values of certain columns.

• Indexes in SQL are used to slow down data retrieval operations on a database table

Explain the purpose of the SQL Data Definition Language (DDL).

• To perform complex calculations on the data

• To retrieve and manipulate data in the database

• To manage user access and security in the database

• To define and manage the structure of the database

How can you update data in a SQL table using the UPDATE statement?

• UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition;

• MODIFY table_name SET column1 = value1, column2 = value2, ... WHERE condition;

• UPDATE table_name VALUES (column1 = value1, column2 = value2, ...) WHERE condition;

• CHANGE table_name SET column1 = value1, column2 = value2, ... WHERE condition;

What is the difference between UNION and UNION ALL in SQL?

• UNION removes duplicates, UNION ALL includes duplicates

• UNION combines data from different tables, UNION ALL combines data from the same table

• UNION sorts the data in ascending order, UNION ALL sorts the data in descending order

• UNION performs an inner join, UNION ALL performs an outer join

How can you calculate the average value of a column in SQL?

• AVG() function

• COUNT() function

• MAX() function

• SUM() function

What are the different types of SQL joins and when would you use each one?

• There are four types of SQL joins: inner join, left join, right join, and full outer join.

• Primary join, secondary join, tertiary join, quaternary join

• Merge join, hash join, loop join, sort join

• Union join, cross join, self join, natural join