T-SQL: Update a table from another table(s)

1.
UPDATE TABLE ASET A.FLD_SUPV = ( SELECT B.FLD_SUPVFROM TABLEA A, TABLEB B, TABLEC C,TABLED DWHERE A.FLD1= B.FLD1AND A.FLD_DT >= B.FLD_FM_DTAND A.FLD_DT <= B.FLD_THRU_DTAND A.FLD_DT > D.FLD_THRU_DTAND A.FLD_DT < fld1=" B.FLD1AND">= B.FLD_FM_DTAND A.FLD_DT <= B.FLD_THRU_DTAND A.FLD_DT > D.FLD_THRU_DTAND A.FLD_DT < C.FLD_EFF_DT )

2.
MERGE INTO TABLEA AUSING (SELECT B.FLD_SUPVFROM TABLEB B, TABLEC C,TABLED DWHERE (A.FLD1= B.FLD1AND A.FLD_DT >= B.FLD_FM_DTAND A.FLD_DT <= B.FLD_THRU_DT)AND A.FLD_DT > D.FLD_THRU_DTAND A.FLD_DT < C.FLD_EFF_DT) AS SON 1=1WHEN MATCHED THENUPDATE SET A.FLD_SUPV = S.FLD_SUPV