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=#