SQL · Theory

UNHEX()()

Theory

174/439

UNHEX()()

UNHEX() is a MySQL string function — it takes text and returns text or a number. You use it in SELECT (and sometimes WHERE/UPDATE). It does not change stored data unless you UPDATE … SET col = UNHEX(col).

Board: SELECT UNHEX(name) FROM students; Run it. Say the result out loud. If the column is NULL, the result is often NULL — wrap with IFNULL if you need a blank instead.

String indexes in MySQL start at 1, not 0 (SUBSTR, INSTR, LEFT). NULL input often yields NULL. Try it on one row first.

Interview — name UNHEX, one argument list, one example on the students table. Don’t recite every string function.

On the example next to this theory: UNHEX()(): create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Exam tip

Say what UNHEX returns, then SELECT UNHEX(name) FROM students;

UNHEX()() — sample query

-- UNHEX()()
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;

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

Short notes

DefUNHEX() is a MySQL string function — it takes text and returns text or a number.
RuleSELECT UNHEX(name) FROM students;
TrapNULL input often yields NULL. Try it on one row first.

SQL · Theory

UNHEX()()

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Theory

174/439

UNHEX()()

Board: SELECT UNHEX(name) FROM students; Run it. Say the result out loud. If the column is NULL, the result is often NULL — wrap with IFNULL if you need a blank instead.

String indexes in MySQL start at 1, not 0 (SUBSTR, INSTR, LEFT). NULL input often yields NULL. Try it on one row first.

Interview — name UNHEX, one argument list, one example on the students table. Don’t recite every string function.

On the example next to this theory: UNHEX()(): create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

Exam tip

Say what UNHEX returns, then SELECT UNHEX(name) FROM students;

UNHEX()() — sample query

-- UNHEX()()
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;

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

Short notes

DefUNHEX() is a MySQL string function — it takes text and returns text or a number.
RuleSELECT UNHEX(name) FROM students;
TrapNULL input often yields NULL. Try it on one row first.

UNHEX()()

UNHEX()() — sample query

Short notes

Questions

UNHEX()()

UNHEX()() — sample query

Short notes

Questions