Bug#32848: Data type conversion bug in union subselects in MySQL 5.0.38
There were two problems when inferring the correct field types resulting from UNION queries. - If the type is NULL for all corresponding fields in the UNION, the resulting type would be NULL, while the type is BINARY(0) if there is just a single SELECT NULL. - If one SELECT in the UNION uses a subselect, a temporary table is created to represent the subselect, and the result type defaults to a STRING type, hiding the fact that the type was unknown(just a NULL value). Fixed by remembering whenever a field was created from a NULL value and pass type NULL to the type coercion if that is the case, and creating a string field as result of UNION only if the type would otherwise be NULL. mysql-test/r/union.result: Bug#32848: Test result mysql-test/t/union.test: Bug#32848: Test case sql/field.cc: Bug#32848: Initialization of new field sql/field.h: Bug#32848: New member to record when a field was created from a NULL value. sql/item.cc: Bug#32848: A field created from a NULL value will submit NULL as type to the type coercion procedure. If Item_type_holder has not inferred the correct type after processing all SELECTs in a UNION, a string field is created. sql/sql_select.cc: Bug#32848: Recording when a field is created from a NULL value.
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