SQL Questions

A subquery is a query nested inside another query, used to fetch data that the main query then uses, effectively acting as a dynamic input for the main query.

A LEFT JOIN returns all rows from the left table (the table mentioned before LEFT JOIN) and the matching rows from the right table. If there are no matches, it fills in NULL values for the columns from the right table.

SQL stands for Structured Query Language. It's a standardized language for managing and interacting with relational databases.

The INSERT statement is used to insert new rows with data into an existing table within a database.

The WHERE clause in SQL acts as a filter, allowing you to select only those rows that meet the condition you define.

UPPER() is a string function that takes a string as an argument and returns the string converted to uppercase.

IN compares a value against a list of values returned by a subquery. EXISTS checks whether the subquery returns any rows at all, regardless of the values.

CURDATE() returns only the current date in 'YYYY-MM-DD' format, while NOW() returns both the current date and time.

MAX() is an aggregate function that returns the maximum value in a set of values (typically a column).

An IN subquery checks if a specific value exists within the set of results returned by the subquery, making it useful for checking against multiple possibilities.

HAVING is used to filter groups after the GROUP BY clause, making it suitable for conditions involving aggregate functions like AVG used for calculating the average salary.

Correlated subqueries execute for each row of the outer query, potentially leading to performance bottlenecks, especially with large datasets. Proper indexing and optimization are crucial.

EXISTS subqueries are generally more efficient when checking for the existence of rows, especially in larger datasets, as it stops evaluation once a single matching row is found.

The OR operator allows rows to be included if they satisfy either one or both of the connected conditions.

The WHERE clause is crucial with UPDATE as it defines which rows should be modified. Without it, all rows would be updated.

The WHERE clause is used to specify conditions that rows must meet to be included in the query results.

COUNT() is an aggregate function used to count the number of rows in a table or the number of rows that meet a specific condition in a query.

The SELECT clause is the core of retrieving data in SQL. It specifies the columns you want to get from a table.

VARCHAR (Variable Character) is used for storing strings of text, making it appropriate for a last name which can vary in length.

In SQL, the asterisk (*) is a shorthand to select all columns from a table when used in the SELECT clause.