Why is the Order of Evaluation Undefined for MySQL Expressions with User Variables?

Why the Order of Evaluation for Expressions Involving User Variables is Undefined

Expressions involving user variables exhibit an undefined order of evaluation in MySQL, as illustrated by the given query. This query potentially produces varying results due to this enigmatic evaluation sequence.

The MySQL manual explicitly states that "the order of evaluation for expressions involving user variables is undefined." This discrepancy stems from the absence of a prescribed evaluation order in the SQL standard. Subsequently, each database system is granted the autonomy to determine its own evaluation strategy.

Typically, this decision is delegated to the optimizer, which seeks to optimize query execution based on various factors. As a result, the sequence of evaluation for expressions involving user variables remains uncertain, leading to unpredictable results in certain instances.

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