Why Does MySQL Treat String Comparisons to Zero as True?

String to Zero Comparison in MySQL: Unveiling the Truth

When comparing string columns with zero in MySQL, a surprising behavior is revealed: the result evaluates to true. This occurs in scenarios like:

select 'string' = 0 as res; -- res = 1 (true)

However, comparing the same string to other numbers, both positive and negative, yields the expected false result. This inconsistency raises the question: why does this anomaly occur?

Behind the String Magic

MySQL quietly converts strings to numbers during comparisons. For strings that do not start with a number, the conversion results in zero. This explains why:

select 'string' = 0 as res;  -- res = 1 (true)

Controlling the Conversion

While MySQL often handles conversions automatically, forcing them using operators like ' ' can be useful. Consider this:

select '0string' + 0 = 'string' AS res; -- res = 1 (true)

In this query, the string '0string' is added to zero, prompting its conversion to a number. Subsequently, the converted string 'string' is compared to zero, again triggering a conversion. The resulting comparison is between numeric values, yielding true.

MySQL's automatic string conversions extend beyond comparisons. For instance:

select '1abc' + '2ef' AS total; -- total = 1+2 = 3

Strings are converted to numbers before addition, resulting in correct numeric operations.

Understanding this string conversion mechanism helps demystify the seemingly paradoxical behavior in MySQL string comparisons. It allows developers to harness this feature effectively in query writing and data manipulation.

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