Why MySQL UPDATE Affected Rows Can Mislead Your Business Logic

In typical business systems, developers use MySQL UPDATE statements and often rely on the number of affected rows to make further decisions, as shown in the pseudo‑code example:

if (xxxMapper.updateByPrimaryKeySelective(entity) > 0) {
    // update succeeded, perform other business processing
}

However, MySQL only reports a positive affected‑row count when the statement actually changes data. If a table contains a single row (ID = 1) and an UPDATE sets the column to its current value, the first execution reports 1 affected row, but a subsequent identical execution reports 0 because no data changed.

This means that when the incoming data matches the existing values, MySQL returns 0, which is indistinguishable from the case where the target row does not exist at all.

Conclusion: do not use the affected‑row count of an UPDATE statement for critical business judgments.

Images illustrate the table before and after the update and the resulting row‑count output.