[MDEV-21076] NOT NULL and UNIQUE constraints cause SUM() to yield an incorrect result - Jira

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Consider the following statements:

CREATE TABLE t0(c0 INT NOT NULL, c1 CHAR UNIQUE);

INSERT INTO t0 VALUES (0, 1);

INSERT INTO t0 VALUES (0, '');

SELECT SUM(a.t) FROM (SELECT (c1 RLIKE c1) = (c0 IS NULL) as t FROM t0) as a; -- expected: 1, actual: 0

I would expect SUM() to yield 1, because the expression evaluates to TRUE for one row:

SELECT (c1 RLIKE c1) = (c0 IS NULL) FROM t0; -- 1, 0