1
Functions are built-in routines that take input values (arguments) and return a result.
2
They use the syntax: FUNCTION_NAME(arg1, arg2, ...).
3
Functions can be used in SELECT, WHERE, GROUP BY, ORDER BY, HAVING, and JOIN conditions.
4
Categories include string, numeric, date/time, aggregate, control-flow, JSON, and more.

Examples of Functions

2. What Are MySQL Operators?

1
Operators are symbols or keywords used to perform calculations, comparisons, or logical evaluations.
2
Operators work between operands (e.g., value1 operator value2).
3
Types include arithmetic, comparison, logical, bitwise, and pattern-matching operators.

Examples of Operators

3. Key Differences Between Functions and Operators

1
Syntax: Functions use parentheses; operators use symbols or keywords.
2
Operation style: Functions execute routines; operators evaluate expressions.
3
Complexity: Functions handle advanced processing; operators perform simpler evaluations.
4
Arguments: Functions accept multiple inputs; operators typically act on two operands.

In summary, functions are callable routines used for data transformation, while operators are symbolic mechanisms used for calculations and comparisons within expressions.

Difficulty: 5/10

Topics: SQL functions, operators, query performance

Scenario Questions

0-2 years experience

1
You need to calculate total price with tax for each order in a SELECT. Would you use a function or an operator, and how would you write that expression?
2
If you write SELECT * FROM users WHERE age + 5 > 30, what MySQL construct is the + and how does it differ from using CONCAT() to combine strings?
3
To get the current date formatted as YYYY‑MM‑DD, which MySQL function would you choose, and could you achieve the same result with an operator?

2-5 years experience

1
Your team added a custom stored function to compute a discount, but query latency increased. How would you determine if the slowdown is due to the function versus using built‑in operators?
2
During a migration, a query that used DATEDIFF() started returning wrong results after the SQL mode changed. Explain why functions and operators can behave differently under mode changes.
3
You need to rewrite a complex WHERE clause that mixes arithmetic operators and string functions for readability. What trade‑offs do you consider when replacing operators with functions or vice‑versa?

5-8 years experience

1
In a high‑traffic reporting service, you must decide between built‑in arithmetic operators and user‑defined functions for on‑the‑fly calculations. How do you evaluate performance, caching, and maintainability?
2
A legacy codebase heavily uses custom functions for simple calculations, causing plan‑cache misses. How would you refactor the queries and what impact on the optimizer would you expect?
3
Explain how MySQL’s operator precedence interacts with function evaluation in a multi‑join query, and how you would debug unexpected results.

8+ years experience

1
Your organization is standardizing SQL style across dozens of services. How would you set guidelines for when to prefer functions over operators, considering readability, portability, and future optimizer changes?
2
During a major version upgrade, several stored functions are deprecated while new operators are introduced. How would you plan the migration to minimize risk and ensure backward compatibility?
3
Design a monitoring strategy to detect performance regressions caused by misuse of functions versus operators in production queries across multiple teams.

Follow-up Questions

❓Can you show a case where an operator is clearly better than a function?
❓How does the optimizer treat functions versus operators in a query plan?
❓What limitations do functions have that operators don’t?