What does the error message "operand data type invalid" mean in an SQL query?

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The error message "operand data type invalid" in an SQL query means that the data type used as an operand in a particular statement is not compatible with the expected data type for that operation. It often happens when you are trying to perform arithmetic, comparison or other operations on columns/variables with different data types. For example, trying to add a text string to a numeric field or attempting to compare a date field with a text string can lead to this error message.