COUNT with subquery fail on H2 database with “Duplicate column name”

问题

we aware strange behavior of H2 when querying COUNT with a subquery.

Prepare table:

CREATE TABLE Foo
(
  id INT PRIMARY KEY AUTO_INCREMENT,
  fieldName VARCHAR(30) NOT NULL,
);

Test simple query (works fine):

SELECT F1.id, F2.id from Foo as F1 INNER JOIN Foo F2 on F1.id = F2.id

Test same query with count:

SELECT count(*) FROM (
  SELECT F1.id, F2.id from Foo as F1 INNER JOIN Foo F2 on F1.id = F2.id
) q;

Got following error:

[42S21][42121] Duplicate column name "ID"; SQL statement: select count(*) FROM (

Any workarounds?

UPDATED: The problem was in that original table name erased when preparing subquery column names and in fact I have :

SELECT count(*) FROM (
  SELECT id, id from q
);

Adding alias (as mentioned by Abdul Rasheed) fix the problem.

回答1:

I don't understand why to use the same field and the same table with aliases. Why don't you use simply a query like that :

SELECT count(id) FROM Foo

回答2:

This problem isn't stricly H2 related, but happens in all SQL databases. A top-level SQL SELECT statement is allowed to project two columns by the same name, but a subquery - in particular a derived table - is not. This is irrespective of the fact whether the ambiguous column name from the subquery is ever referenced.

The solution is to explicitly alias your columns:

SELECT count(*) FROM (
  SELECT f1.id AS f1_id, f2.id AS f2_id 
  FROM foo AS f1 
  INNER JOIN foo f2 ON f1.id = f2.id
) q;

Or to avoid the projection in the derived table entirely, as you're not going to need it to calculate the count value:

SELECT count(*) FROM (
  SELECT 1
  FROM foo AS f1 
  INNER JOIN foo f2 ON f1.id = f2.id
) q;

At this point, you could obviously just avoid the derived table completely:

SELECT count(*) 
FROM foo AS f1 
INNER JOIN foo f2 ON f1.id = f2.id;

An alternative when using H2 1.4.198 or later is to use window functions to calculate the count value on each row:

SELECT f1.id AS f1_id, f2.id AS f2_id, count(*) OVER ()
FROM foo AS f1 
INNER JOIN foo f2 ON f1.id = f2.id

来源：https://stackoverflow.com/questions/47282769/count-with-subquery-fail-on-h2-database-with-duplicate-column-name

标签

sql