Year format is different on different tables
The format of the YR column is different on many tables, making joins complicated. An example is ENR, which uses an integer year, and AHS, which uses an XXXX-YYYY format year. There is at least one other table that uses XXXX-YY.
Chris Holloway commented
Another interesting example is HIS.YR, which is stored as YY instead of YYYY as in ENR.YR