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There can be various reasons for the WHERE clause in MySQL to produce zero rows, some of them are:

1. No matching values: If the WHERE clause specifies certain conditions for selecting rows, but none of the rows in the table meet the conditions, then the result set will be empty.

2. Incorrect syntax: If the WHERE clause contains incorrect syntax, such as misspelled column names or incorrect operators, then the query will not be able to execute correctly and may produce zero rows.

3. Null values: If the WHERE clause contains conditions that involve null values, such as "WHERE columnname = NULL" instead of "WHERE columnname IS NULL", it may produce zero rows even if there are null values in the table.

4. Table or schema permissions: If the user executing the query does not have sufficient permissions to access the table or schema in question, then the query may produce zero rows.

5. Data type mismatch: If the WHERE clause specifies conditions that involve incompatible data types, such as trying to compare a string with an integer, then the query may produce zero rows.