Function string_to_sql 

pub fn string_to_sql(
    ty: FieldType,
    case: Option<TextCase>,
    s: &str,
) -> Result<Value, String>

Convert a raw query-string value (always a string) into a typed SQL value for a column, used for ?field=value filtering.

The filter is cased the same way the column is, so ?currency=eur finds the rows stored as EUR. A filter that had to be spelled the way the storage happens to be is a filter that silently returns nothing.