This post was prompted by Hans-Juergen Schoenig’s Common mistakes: UNION vs. UNION ALL because it touches on one of my pet peeves: the claim that some feature of SQL exemplifies or conforms to the relational model. Schoenig does not make that claim explicitly, but he does state “What [most] people in many cases really want is UNION ALL” and shows the following query and result:
test=# SELECT 1 UNION ALL SELECT 1; ?column? ---------- 1 1 (2 rows)
There are two relational faults above*. First, UNION ALL is not a relational operator. This is an area where both Ted Codd and Chris Date (and Hugh Darwen), are fully in agreement. In the “Serious Flaws in SQL” chapter of The Relational Model for Database Management: Version 2 (1990) Codd listed duplicate rows as the first flaw and characterized “relations in which duplicate rows are permitted as corrupted relations.” Date concurs and wrote the essay “Why Duplicate Rows Are Prohibited”(in Relational Database Writings, 1985-1989) and (with Darwen) included RM Proscription 3: No Duplicate Tuples in their Third Manifesto, which reads:
D shall include no concept of a “relation” containing two distinct tuples t1 and t2 such that the comparison “t1 = t2” evaluates to TRUE. It follows that (as already stated in RM Proscription 2), for every relation r expressible in D, the tuples of r shall be distinguishable by value.
Needless to say, those two “1”s are not distinguishable unless you talk about “the first 1” and “the last 1,” i.e., ordering, which is also proscribed by the relational model because relations are sets.
Now, the example given is synthetic so I’ll present a more realistic example. Suppose a manager asks “which employees are in department 51 or work on the Skunk Works project?” Let’s assume we have a projects table with columns proj_no (primary key) and proj_name, an emp table with columns emp_no (primary key), last_name, first_name, and dept_no, and an assignments table with columns proj_no and emp_no (both forming the primary key and each referencing the previous two tables, respectively). We’ll first emulate this with a CTE, so we won’t have to create or populate any tables:
WITH emp AS (SELECT 'Ben Rich'::text AS emp_name, 51 AS dept_no), assignments AS (SELECT 'Ben Rich'::text AS emp_name, 'Skunk Works'::text AS proj_name) SELECT emp_name FROM emp WHERE dept_no = 51 UNION ALL SELECT emp_name FROM assignments WHERE proj_name = 'Skunk Works';
If you run this in psql, you’ll see two rows with identical values and the manager is going to ask “Do we have two employees named Ben Rich?” However, in practice the real query will be:
SELECT first_name, last_name FROM emp WHERE dept_no = 51 UNION ALL SELECT first_name, last_name FROM emp JOIN assignments USING (emp_no) JOIN projects p USING (proj_no) WHERE p.proj_name = 'Skunk Works';
Unless you change UNION ALL to UNION your result wil contain duplicate rows for employees that satisfy both predicates. However, an alternative formulation without UNION would be
SELECT first_name, last_name FROM emp LEFT JOIN assignments USING (emp_no) LEFT JOIN projects p USING (proj_no) WHERE dept_no = 51 OR p.proj_name = 'Skunk Works';
This query correctly returns only one row per employee. Admittedly, the query is still somewhat synthetic. In reality, the query may include multiple other columns and several hundred rows may be retrieved and thus the duplicate tuples and the logical error may not be so obvious.
UPDATE: Changed last query to use LEFT JOINs as correctly suggested by RobJ below.
* The second relational fault? The result column is unnamed (something Date and Darwen insist on much more than Codd).