PostgreSQL/Postgre SQL/If

Using the IF/THEN/ELSE statement

postgres=#
postgres=# CREATE TABLE "editions" (
postgres(#      "isbn" text NOT NULL,
postgres(#      "book_id" integer,
postgres(#      "edition" integer,
postgres(#      "publisher_id" integer,
postgres(#      "publication" date,
postgres(#      "type" character(1)
postgres(# );
ERROR:  relation "editions" already exists
postgres=#
postgres=# insert into editions values("039480001X", 1608,  1,  59,  "1957-03-01", "h");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0451160916", 7808,  1,  75,  "1981-08-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0394800753", 1590,  1,  59,  "1949-03-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0590445065", 25908, 1,  150, "1987-03-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0694003611", 1501,  1,  65,  "1947-03-04", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0679803335", 1234,  1,  102, "1922-01-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0760720002", 190,   1,  91,  "1868-01-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0394900014", 1608,  1,  59,  "1957-01-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0385121679", 7808,  2,  75,  "1993-10-01", "h");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("1885418035", 156,   1,  163, "1995-03-28", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0929605942", 156,   2,  171, "1998-12-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0441172717", 4513,  2,  99,  "1998-09-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("044100590X", 4513,  3,  99,  "1999-10-01", "h");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0451457994", 4267,  3,  101, "2000-09-12", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0451198492", 4267,  3,  101, "1999-10-01", "h");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0823015505", 2038,  1,  62,  "1958-01-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=# insert into editions values("0596000855", 41473, 2,  113, "2001-03-01", "p");
ERROR:  duplicate key violates unique constraint "pkey"
postgres=#
postgres=# select * from editions;
    isbn    | book_id | edition | publisher_id | publication | type
------------+---------+---------+--------------+-------------+------
 039480001X |    1608 |       1 |           59 | 1957-03-01  | h
 0451160916 |    7808 |       1 |           75 | 1981-08-01  | p
 0394800753 |    1590 |       1 |           59 | 1949-03-01  | p
 0590445065 |   25908 |       1 |          150 | 1987-03-01  | p
 0694003611 |    1501 |       1 |           65 | 1947-03-04  | p
 0679803335 |    1234 |       1 |          102 | 1922-01-01  | p
 0760720002 |     190 |       1 |           91 | 1868-01-01  | p
 0394900014 |    1608 |       1 |           59 | 1957-01-01  | p
 0385121679 |    7808 |       2 |           75 | 1993-10-01  | h
 1885418035 |     156 |       1 |          163 | 1995-03-28  | p
 0929605942 |     156 |       2 |          171 | 1998-12-01  | p
 0441172717 |    4513 |       2 |           99 | 1998-09-01  | p
 044100590X |    4513 |       3 |           99 | 1999-10-01  | h
 0451457994 |    4267 |       3 |          101 | 2000-09-12  | p
 0451198492 |    4267 |       3 |          101 | 1999-10-01  | h
 0823015505 |    2038 |       1 |           62 | 1958-01-01  | p
 0596000855 |   41473 |       2 |          113 | 2001-03-01  | p
(17 rows)
postgres=#
postgres=#
postgres=#
postgres=# -- Using the IF/THEN/ELSE statement
postgres=#
postgres=# CREATE FUNCTION in_stock (integer,integer) RETURNS boolean AS "
postgres"#   DECLARE
postgres"#      -- Declare aliases for function arguments.
postgres"#     b_id ALIAS FOR $1;
postgres"#     b_edition ALIAS FOR $2;
postgres"#
postgres"#      -- Declare a text variable to hold the ISBN of the book
postgres"#      -- once found.
postgres"#     b_isbn TEXT;
postgres"#
postgres"#      -- Declare an integer variable to hold the amount of stock.
postgres"#     stock_amount INTEGER;
postgres"#
postgres"#   BEGIN
postgres"#
postgres"#     SELECT INTO b_isbn isbn FROM editions WHERE
postgres"#       book_id = b_id AND edition = b_edition;
postgres"#
postgres"#     IF b_isbn IS NULL THEN
postgres"#       RETURN FALSE;
postgres"#     END IF;
postgres"#
postgres"#     SELECT INTO stock_amount stock FROM stock WHERE isbn = b_isbn;
postgres"#
postgres"#     IF stock_amount <= 0 THEN
postgres"#       RETURN FALSE;
postgres"#     ELSE
postgres"#       RETURN TRUE;
postgres"#     END IF;
postgres"#
postgres"#   END;
postgres"# " LANGUAGE "plpgsql";
CREATE FUNCTION
postgres=#
postgres=#
postgres=# SELECT in_stock(4513,2);
 in_stock
----------
 t
(1 row)
postgres=#
postgres=#
postgres=# drop table editions;
DROP TABLE
postgres=#