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CREATE TABLE t1 (f1 INTEGER PRIMARY KEY , f2 VARCHAR(0)) ENGINE=InnoDB;
CREATE TABLE t2 (f1 VARCHAR(0)) ENGINE=InnoDB;
INSERT INTO t1 VALUES (1, NULL);
INSERT INTO t1 VALUES (2, '');
INSERT INTO t2 VALUES (NULL);
INSERT INTO t2 VALUES ('');
SELECT COUNT(*) = 2 FROM t1;
COUNT(*) = 2
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SELECT f2 IS NULL FROM t1 WHERE f1 = 1;
f2 IS NULL
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SELECT f2 = '' FROM t1 WHERE f1 = 2;
f2 = ''
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SELECT COUNT(*) = 2 FROM t2;
COUNT(*) = 2
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SELECT f1 IS NULL FROM t2 WHERE f1 IS NULL;
f1 IS NULL
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SELECT f1 = '' FROM t2 WHERE f1 IS NOT NULL;
f1 = ''
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UPDATE t1 SET f2 = '' WHERE f1 = 1;
UPDATE t1 SET f2 = NULL WHERE f1 = 2;
UPDATE t2 SET f1 = '' WHERE f1 IS NULL;
SELECT f2 = '' FROM t1 WHERE f1 = 1;
f2 = ''
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SELECT f2 IS NULL FROM t1 WHERE f1 = 2;
f2 IS NULL
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SELECT COUNT(*) = 2 FROM t2 WHERE f1 = '';
COUNT(*) = 2
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DROP TABLE t1;
DROP TABLE t2;
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