On 22/07/11 13:20, Chris Curvey wrote:
in an earlier thread, the following query was submitted:
SELECT COUNT(*) FROM q
WHERE NOT EXISTS (SELECT 1
FROM t AS t
WHERE t.mid = q.mid);
and as part of his answer, David Johnson responded
3. Also, you are using a correlated sub-query instead of a LEFT OUTER JOIN
This interests me. How would you rewrite the correlated subquery above using the outer join?
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DROP TABLE IF EXISTS q;
DROP TABLE IF EXISTS t;
CREATE TABLE q
(
mid int PRIMARY KEY,
qnote text
);
CREATE TABLE t
(
mid int PRIMARY KEY,
tnote text
);
INSERT INTO q (mid, qnote) VALUES
(1, 'first'),
(2, 'second'),
(4, 'fourth');
INSERT INTO t (mid, tnote) VALUES
(1, 'first'),
(3, 'third');
SELECT
COUNT(*)
FROM
q LEFT OUTER JOIN t ON (q.mid = t.mid)
WHERE
t.mid IS NULL;
