EC
3204
MySQLERRORNotableDDLHIGH confidence

Invalid default expression for generated column

Production Risk

Low — DDL error.

Why it happens
  1. 1Specifying an explicit DEFAULT value on a generated column, which is not permitted.
  2. 2Generated columns derive their value from the expression; no DEFAULT is allowed.
How to reproduce
trigger — this will error
trigger — this will error
ALTER TABLE t1 ADD COLUMN gc INT AS (col1 * 2) VIRTUAL DEFAULT 0;

expected output

ERROR 3204 (HY000): Generated column cannot have a default value.

Fix

Remove the DEFAULT clause

Remove the DEFAULT clause
ALTER TABLE t1 ADD COLUMN gc INT AS (col1 * 2) VIRTUAL;

Why this works

Generated columns do not accept DEFAULT; their value is computed.

What not to do

✕

Sources
Official documentation ↗

MySQL 8.0 — 3204 ER_INVALID_DEFAULT_EXPR_GENERATED_COLUMN

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