SQL Mastery

What is the purpose of the SQL SELECT statement?

• To delete data from a database

• To create a new database

• To retrieve data from a database

• To update data in a database

Explain the difference between the WHERE and HAVING clauses in SQL.

• The WHERE clause is used to filter rows before any groupings are made, while the HAVING clause is used to filter rows after the grouping has been done.

• The WHERE clause is used for updating data, while the HAVING clause is used for inserting new data.

• The WHERE clause is used for joining tables, while the HAVING clause is used for sorting the result set.

• The WHERE clause is used for mathematical operations, while the HAVING clause is used for string manipulations.

What are the different types of SQL joins? Provide an example for each.

• UPPER JOIN, LOWER JOIN, MIDDLE JOIN

• OUTER JOIN, CENTER JOIN, SIDE JOIN

• INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN

• FIRST JOIN, SECOND JOIN, THIRD JOIN

How does the UNION operator work in SQL? Provide an example.

• The UNION operator in SQL is used to divide the result sets of two or more SELECT statements.

• The UNION operator in SQL is used to multiply the result sets of two or more SELECT statements.

• The UNION operator in SQL is used to subtract the result sets of two or more SELECT statements.

• The UNION operator in SQL is used to combine the result sets of two or more SELECT statements.

What are the commonly used aggregation functions in SQL? Provide a brief explanation for each.

• HIGH, LOW, RANGE, MEDIAN

• ADD, SUBTRACT, MULTIPLY, DIVIDE

• COUNT, SUM, AVG, MIN, and MAX

• FIRST, LAST, AVERAGE, TOTAL

Explain the concept of primary key in SQL Data Definition Language.

• A primary key in SQL Data Definition Language is a column or a set of columns that uniquely identifies each row in a table.

• A primary key in SQL is used to perform mathematical calculations on the data in the table.

• Primary key in SQL is a column that allows duplicate values in a table.

• Primary key in SQL is used to store large amounts of text data in a table.

What is the purpose of the SQL UPDATE statement? Provide an example.

• To insert new records into a table. Example: UPDATE table_name VALUES (value1, value2)

• To create a new table in the database. Example: UPDATE new_table_name

• To modify existing records in a table. Example: UPDATE table_name SET column1 = value1 WHERE condition;

• To delete all records in a table. Example: UPDATE table_name

Explain the concept of foreign key in SQL Data Definition Language.

• A foreign key in SQL DDL is a field in a table that is a non-key attribute in another table.

• A foreign key in SQL DDL is a field in a table that is a secondary key in another table.

• A foreign key in SQL DDL is a field in a table that is a primary key in another table.

• A foreign key in SQL DDL is a field in a table that is a unique key in another table.

What is the purpose of the SQL GROUP BY clause? Provide an example.

• To filter rows before any groupings are made. Example: GROUP BY column_name

• To filter rows after the grouping has been done. Example: GROUP BY HAVING column_name

• To combine the result sets of two or more SELECT statements. Example: GROUP BY UNION column_name

• To sort the result set. Example: GROUP BY ORDER BY column_name

What is the purpose of the SQL DISTINCT keyword? Provide an example.

• The DISTINCT keyword in SQL is used to select unique values from a column. Example: SELECT DISTINCT column_name FROM table_name

• The DISTINCT keyword in SQL is used to delete duplicate rows from a table. Example: DELETE FROM table_name WHERE column_name = value

• The DISTINCT keyword in SQL is used to update specific rows in a table. Example: UPDATE table_name SET column_name = value WHERE condition

• The DISTINCT keyword in SQL is used to create a new table with unique values. Example: CREATE TABLE new_table_name AS SELECT DISTINCT * FROM table_name