SQL questions

SQL Injection occurs when an attacker inserts malicious SQL code into a query, which can compromise the database. Prevent SQL Injection by using parameterized queries or prepared statements, validating and escaping user input, and implementing robust input validation and sanitization practices.

`EXISTS` is used in SQL to test for the existence of rows returned by a subquery. It returns `TRUE` if the subquery returns one or more rows and `FALSE` otherwise. For example: `SELECT * FROM employees WHERE EXISTS (SELECT 1 FROM departments WHERE departments.dept_id = employees.dept_id);` checks if there are corresponding departments for employees.

User-defined functions (UDFs) allow users to create custom functions to encapsulate reusable logic in SQL. UDFs can take parameters and return values. For example, a UDF to calculate tax could be defined as `CREATE FUNCTION CalculateTax(amount DECIMAL) RETURNS DECIMAL BEGIN RETURN amount * 0.1; END;`.

The CASE statement allows conditional logic in SQL queries, returning values based on specified conditions. It works like an IF statement. For example, `SELECT name, CASE WHEN score >= 60 THEN 'Pass' ELSE 'Fail' END AS result FROM exams;` assigns 'Pass' or 'Fail' based on the score.

Key differences between SQL databases and MongoDB include data structure, schema, and query language. SQL uses tables with fixed schemas, while MongoDB uses collections of documents with flexible schemas. SQL queries are structured in SQL language, whereas MongoDB uses a JSON-like query syntax, making it more intuitive for developers familiar with JSON.