aTest <- data.frame(AccountNumber = c("10001", "20002", "40004"),
               ServiceProvider = c("Jabo Gas and Electric",
                                   "Java Coffee", "Java Coffee"),
               DBID = c("001", "002", "003"))

bTest <-data.frame(AccountNumber = c("10001", "20002", "30003", "40004"),
              Installation = c("Base A", "Base B", "Base C", "Base D"),
              ProviderID = c("H12", "J02", "K03", "L09"))

# Outer join: merge(x = aTest, y = bTest, by = "AcountNumber", all = TRUE)
#
# > merge(x = aTest, y = bTest, by = "AccountNumber", all = TRUE)
# AccountNumber       ServiceProvider DBID Installation ProviderID
# 1         10001 Jabo Gas and Electric  001       Base A        H12
# 2         20002           Java Coffee  002       Base B        J02
# 3         40004           Java Coffee  003       Base D        L09
# 4         30003                  <NA> <NA>       Base C        K03
#
# Left outer: merge(x = aTest, y = bTest, by = "AcountNumber", all.x = TRUE)
#
# > merge(x = aTest, y = bTest, by = "AccountNumber", all.x = TRUE)
# AccountNumber       ServiceProvider DBID Installation ProviderID
# 1         10001 Jabo Gas and Electric  001       Base A        H12
# 2         20002           Java Coffee  002       Base B        J02
# 3         40004           Java Coffee  003       Base D        L09
#
# Right outer: merge(x = aTest, y = bTest, by = "AcountNumber", all.y = TRUE)
#
# > merge(x = aTest, y = bTest, by = "AccountNumber", all.y = TRUE)
# AccountNumber       ServiceProvider DBID Installation ProviderID
# 1         10001 Jabo Gas and Electric  001       Base A        H12
# 2         20002           Java Coffee  002       Base B        J02
# 3         40004           Java Coffee  003       Base D        L09
# 4         30003                  <NA> <NA>       Base C        K03
#
# Cross join: merge(x = aTest, y = bTest, by = NULL)
#
# > merge(x = aTest, y = bTest, by = NULL)
# AccountNumber.x       ServiceProvider DBID AccountNumber.y Installation ProviderID
# 1            10001 Jabo Gas and Electric  001           10001       Base A        H12
# 2            20002           Java Coffee  002           10001       Base A        H12
# 3            40004           Java Coffee  003           10001       Base A        H12
# 4            10001 Jabo Gas and Electric  001           20002       Base B        J02
# 5            20002           Java Coffee  002           20002       Base B        J02
# 6            40004           Java Coffee  003           20002       Base B        J02
# 7            10001 Jabo Gas and Electric  001           30003       Base C        K03
# 8            20002           Java Coffee  002           30003       Base C        K03
# 9            40004           Java Coffee  003           30003       Base C        K03
# 10           10001 Jabo Gas and Electric  001           40004       Base D        L09
# 11           20002           Java Coffee  002           40004       Base D        L09
# 12           40004           Java Coffee  003           40004       Base D        L09