no-null-out-column

Reports an UPDATE that only clears columns, setting them to NULL on an existing table, which deletes the values they held.

Category	Default severity	Databases
data-loss	warn	PostgreSQL, MySQL

What happens

UPDATE ... SET "column" = NULL overwrites the column's values once the migration commits. down() cannot bring them back unless the migration saved a copy first. The finding names the column and, on PostgreSQL, the WHERE condition, or says that every row is changed.

An UPDATE that also changes other columns is not reported here, because a cleared column there is usually a side effect, such as resetting updated_by. rewrites-existing-rows lists that statement instead, and names the columns it clears.

A table created in the same migration is not reported: it has no values to lose.

Bad

import { MigrationInterface, QueryRunner } from 'typeorm'

export class ClearUkPhones1727200000000 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`UPDATE "users" SET "phone" = NULL WHERE "country" = 'UK'`)
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    // ...
  }
}

Safe

Keep a copy of the values first, so down() can restore them:

import { MigrationInterface, QueryRunner } from 'typeorm'

export class ClearUkPhones1727200000000 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(
      `CREATE TABLE "users_phone_1727200000000" AS SELECT "id", "phone" FROM "users" WHERE "country" = 'UK'`,
    )
    // preflight safety-assured no-null-out-column -- a copy is kept in users_phone_1727200000000
    await queryRunner.query(`UPDATE "users" SET "phone" = NULL WHERE "country" = 'UK'`)
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(
      `UPDATE "users" u SET "phone" = c."phone" FROM "users_phone_1727200000000" c WHERE u."id" = c."id"`,
    )
    await queryRunner.query(`DROP TABLE "users_phone_1727200000000"`)
  }
}

When to suppress

When the values must go, for example personal data that may no longer be kept, and nothing needs them back. Write that reason:

// preflight safety-assured no-null-out-column -- phone numbers may not be kept after account closure

References

• PostgreSQL: UPDATE ("UPDATE changes the values of the specified columns in all rows that satisfy the condition.")
• MySQL: UPDATE statement