Original query:

SELECT 

    "R"."Lat" AS "Lat",
    "R"."Long R" AS "Long R",
    "R"."Dpt" AS "Dpt",
    "R"."Reg" AS "Reg",

    "B3"."M" AS "M",
    "B3"."St" AS "St",
    "B3"."Sp"AS "Sp",
    "B3"."Reg" AS "Reg",
    "B3"."year" AS "year",
    "B3"."id_program" AS "id_program",
    "B3"."program" AS "program",
    "B3"."Lib" AS "Lib",
    "B3"."Ef"/"R2"."NB Dept" AS "Agg_Ef",
    "B3"."Eex"/"R2"."NB Dept" AS "Agg_Eex",
    "B3"."Ein"/"R2"."NB Dept" AS "Agg_Ein",
    "B3"."Sehr"/"R2"."NB Dept" AS "Agg_Sehr",
    "B3"."Sin"/"R2"."NB Dept" AS "Agg_Sin",
    "B3"."Sr"/"R2"."NB Dept" AS "Agg_Sr",
    "B3"."Sc"/"R2"."NB Dept" AS "Agg_Sr",

    "C2"."RE"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_RE",
    "C2"."RD"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_RD",
    "C2"."RDP"/("R2"."NB Dept"*"B2"."NB St")AS "Agg_RDP",
    "C2"."RC"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_RC",
    "C2"."RA"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_RA",
    "C2"."EE"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_EE",
    "C2"."BE"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_BE",
    "C2"."BD"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_BD",
    "C2"."BDP"/("R2"."NB Dept"*"B2"."NB St")AS "Agg_BDP",
    "C2"."BC"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_BC",
    "C2"."BA"/("R2"."NB Dept"*"B2"."NB St") AS "Agg_BA"

FROM "Database"."R_Table" "R"

    INNER JOIN (
     SELECT
     "R"."Reg" AS "Reg",
     COUNT (DISTINCT "R"."Dpt") AS "NB Dept"
    FROM "Database"."R_Table" "R"  
     GROUP BY "R"."Reg") "R2" on "R2"."Reg" = "R"."Reg"

    INNER JOIN (
     SELECT
     "B"."Reg" AS "Reg",
     COUNT (DISTINCT "B"."St") AS "NB St"
    FROM "Database"."B_Table" "B"  
     GROUP BY "B"."Reg") "B2" on "B2"."Reg" = "R"."Reg"

   INNER JOIN (
    SELECT
    "B"."Job" AS "Job",
    "B"."St" AS "St",
    "B"."Sp" AS "Sp",
    "B"."Reg" AS "Reg",
    "B"."year" AS "year",
    "B"."id_program" AS "id_program",
    "B"."program" AS "program",
    "B"."Lib" AS "Lib",
    SUM("B"."Ef") AS "Ef",
    SUM("B"."Eex") AS "Eex",
    SUM("B"."Ein") AS "Ein",
    SUM("B"."Sehr") AS "Sehr",
    SUM("B"."Sin") AS "Sin",
    SUM("B"."Sr") AS "Sr",
    SUM("B"."Sc") AS "Sc"
  FROM "Database"."B_Table" "B"
    GROUP BY "B"."id_program","B"."program","B"."year","B"."Reg","B"."Job",
             "B"."Sp","B"."Lib","B"."St") "B3" on "B3"."Reg" = "R"."Reg"

   LEFT JOIN (
    SELECT
    "C"."Job" AS "Job",
    "C"."Sp" AS "Sp",
    "C"."Reg" AS "Reg",
    "C"."year" AS "year",
    "C"."id_program" AS "id_program",
    "C"."program" AS "program",
    "C"."Lib" AS "Lib",
    SUM("C"."RE") AS "RE",
    SUM("C"."RD") AS "RE",
    SUM("C"."RDP") AS "RDP",
    SUM("C"."RC") AS "RC",
    SUM("C"."RA") AS "RA",
    SUM("C"."EE") AS "EE",
    SUM("C"."BE") AS "BE",
    SUM("C"."BD") AS "BD",
    SUM("C"."BDP") AS "BPE",
    SUM("C"."BC") AS "BC",
    SUM("C"."BA") AS "BA"
  FROM "Database"."Content" "C"
    GROUP BY "C"."id_program","C"."program","C"."year",
             "C"."Reg","C"."Job", "C"."Sp","C"."Lib") "C2" on  
             concat("C2"."id_program","C2"."program","C2"."year","C2"."Reg","C2"."Job","C2"."Sp","C2"."Lib") = 
             concat("B3"."id_program","B3"."program","B3"."year","B3"."Reg","B3"."Job","B3"."Sp","B3"."Lib")

Prettified:

SELECT
  "R"."Lat" AS Lat,
  "R"."Long R" AS Long R,
  "R"."Dpt" AS Dpt,
  "R"."Reg" AS Reg,
  "B3"."M" AS M,
  "B3"."St" AS St,
  "B3"."Sp" AS Sp,
  "B3"."Reg" AS Reg,
  "B3"."year" AS year,
  "B3"."id_program" AS id_program,
  "B3"."program" AS program,
  "B3"."Lib" AS Lib,
  "B3"."Ef" / "R2"."NB Dept" AS Agg_Ef,
  "B3"."Eex" / "R2"."NB Dept" AS Agg_Eex,
  "B3"."Ein" / "R2"."NB Dept" AS Agg_Ein,
  "B3"."Sehr" / "R2"."NB Dept" AS Agg_Sehr,
  "B3"."Sin" / "R2"."NB Dept" AS Agg_Sin,
  "B3"."Sr" / "R2"."NB Dept" AS Agg_Sr,
  "B3"."Sc" / "R2"."NB Dept" AS Agg_Sr,
  "C2"."RE" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_RE,
  "C2"."RD" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_RD,
  "C2"."RDP" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_RDP,
  "C2"."RC" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_RC,
  "C2"."RA" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_RA,
  "C2"."EE" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_EE,
  "C2"."BE" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_BE,
  "C2"."BD" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_BD,
  "C2"."BDP" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_BDP,
  "C2"."BC" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_BC,
  "C2"."BA" / ("R2"."NB Dept" * "B2"."NB St") AS Agg_BA
FROM
  "Database"."R_Table" R
  JOIN (
    SELECT
      "R"."Reg" AS Reg,
      count(DISTINCT "R"."Dpt") AS NB Dept
    FROM
      "Database"."R_Table" R
    GROUP BY
      "R"."Reg"
  ) R2 ON ("R2"."Reg" = "R"."Reg")
  JOIN (
    SELECT
      "B"."Reg" AS Reg,
      count(DISTINCT "B"."St") AS NB St
    FROM
      "Database"."B_Table" B
    GROUP BY
      "B"."Reg"
  ) B2 ON ("B2"."Reg" = "R"."Reg")
  JOIN (
    SELECT
      "B"."Job" AS Job,
      "B"."St" AS St,
      "B"."Sp" AS Sp,
      "B"."Reg" AS Reg,
      "B"."year" AS year,
      "B"."id_program" AS id_program,
      "B"."program" AS program,
      "B"."Lib" AS Lib,
      sum("B"."Ef") AS Ef,
      sum("B"."Eex") AS Eex,
      sum("B"."Ein") AS Ein,
      sum("B"."Sehr") AS Sehr,
      sum("B"."Sin") AS Sin,
      sum("B"."Sr") AS Sr,
      sum("B"."Sc") AS Sc
    FROM
      "Database"."B_Table" B
    GROUP BY
      "B"."id_program",
      "B"."program",
      "B"."year",
      "B"."Reg",
      "B"."Job",
      "B"."Sp",
      "B"."Lib",
      "B"."St"
  ) B3 ON ("B3"."Reg" = "R"."Reg")
  LEFT JOIN (
    SELECT
      "C"."Job" AS Job,
      "C"."Sp" AS Sp,
      "C"."Reg" AS Reg,
      "C"."year" AS year,
      "C"."id_program" AS id_program,
      "C"."program" AS program,
      "C"."Lib" AS Lib,
      sum("C"."RE") AS RE,
      sum("C"."RD") AS RE,
      sum("C"."RDP") AS RDP,
      sum("C"."RC") AS RC,
      sum("C"."RA") AS RA,
      sum("C"."EE") AS EE,
      sum("C"."BE") AS BE,
      sum("C"."BD") AS BD,
      sum("C"."BDP") AS BPE,
      sum("C"."BC") AS BC,
      sum("C"."BA") AS BA
    FROM
      "Database"."Content" C
    GROUP BY
      "C"."id_program",
      "C"."program",
      "C"."year",
      "C"."Reg",
      "C"."Job",
      "C"."Sp",
      "C"."Lib"
  ) C2 ON (concat("C2"."id_program", "C2"."program", "C2"."year", "C2"."Reg", "C2"."Job", "C2"."Sp", "C2"."Lib") = concat("B3"."id_program", "B3"."program", "B3"."year", "B3"."Reg", "B3"."Job", "B3"."Sp", "B3"."Lib"))