COALESCE() — SQL Technical Interview Questions & Answers

COALESCE()

COALESCE()

  • COALESCE() is a MySQL topic you learn with one small college example — a students table with roll, name, marks. Say what COALESCE() does in one sentence, then write the shortest SQL that shows it.
  • Pattern: USE college; then a CREATE/SELECT/UPDATE that needs COALESCE(). If it is a function, SELECT it on one column. If it is DDL, run SHOW / DESCRIBE after so you can see the change.
  • Trap: only reciting the heading COALESCE() with no SQL. Interviewers ask you to type. Another trap: mixing this up with a neighbouring command (DELETE vs TRUNCATE vs DROP, WHERE vs HAVING, JOIN vs UNION).
  • Keep the example on paper: two or three rows. Change one value. Predict the result before you run it.
  • On the example next to this theory: COALESCE(): create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Technical Interview Tip: What is COALESCE()? One sentence + one tiny SQL on students.

Code Examples for COALESCE()

COALESCE() — sample query

-- COALESCE()
CREATE TABLE demo (
  id INTEGER PRIMARY KEY,
  label TEXT NOT NULL,
  qty INTEGER DEFAULT 0
);
INSERT INTO demo (id, label, qty) VALUES
  (1, 'alpha', 2),
  (2, 'beta', 5);
SELECT label, qty FROM demo WHERE qty >= 2 ORDER BY qty DESC;

COALESCE(): create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

How to Explain COALESCE() to an Interviewer

What is COALESCE()?

A MySQL idea used with tables/queries Show one short SQL

Give an example.

students table SQL that uses COALESCE()

What mistake do beginners make?

Name only, no SQL Mixing nearby commands