1) Replace the missing death date with a much later date
intck('day', dt_discharge,coalesce(dt_death,"31dec2100"d)) le 30 as flg_30d ,
2) Refer to previously calculated values with the keyword calculated
calculated flg_30d or calculated flg_rf as flg_x_30d
3) More detail needed. Where is that re-admission date in your data? I suggest that you post as a new question.
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