NULL 语义

-- Normal comparison operators return `NULL` when one of the operand is `NULL`.
SELECT 5 > null AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             null|
+-----------------+

-- Normal comparison operators return `NULL` when both the operands are `NULL`.
SELECT null = null AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             null|
+-----------------+

-- Null-safe equal operator return `False` when one of the operand is `NULL`
SELECT 5 <=> null AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|            false|
+-----------------+

-- Null-safe equal operator return `True` when one of the operand is `NULL`
SELECT NULL <=> NULL;
+-----------------+
|expression_output|
+-----------------+
|             true|
+-----------------+

-- Normal comparison operators return `NULL` when one of the operands is `NULL`.
SELECT (true OR null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             true|
+-----------------+

-- Normal comparison operators return `NULL` when both the operands are `NULL`.
SELECT (null OR false) AS expression_output
+-----------------+
|expression_output|
+-----------------+
|             null|
+-----------------+

-- Null-safe equal operator returns `False` when one of the operands is `NULL`
SELECT NOT(null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             null|
+-----------------+

SELECT concat('John', null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             null|
+-----------------+

SELECT positive(null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             null|
+-----------------+

SELECT to_date(null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             null|
+-----------------+

SELECT isnull(null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             true|
+-----------------+

-- Returns the first occurrence of non `NULL` value.
SELECT coalesce(null, null, 3, null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|                3|
+-----------------+

-- Returns `NULL` as all its operands are `NULL`. 
SELECT coalesce(null, null, null, null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|             null|
+-----------------+

SELECT isnan(null) AS expression_output;
+-----------------+
|expression_output|
+-----------------+
|            false|
+-----------------+

-- `count(*)` does not skip `NULL` values.
SELECT count(*) FROM person;
+--------+
|count(1)|
+--------+
|       7|
+--------+

-- `NULL` values in column `age` are skipped from processing.
SELECT count(age) FROM person;
+----------+
|count(age)|
+----------+
|         5|
+----------+

-- `count(*)` on an empty input set returns 0. This is unlike the other
-- aggregate functions, such as `max`, which return `NULL`.
SELECT count(*) FROM person where 1 = 0;
+--------+
|count(1)|
+--------+
|       0|
+--------+

-- `NULL` values are excluded from computation of maximum value.
SELECT max(age) FROM person;
+--------+
|max(age)|
+--------+
|      50|
+--------+

-- `max` returns `NULL` on an empty input set.
SELECT max(age) FROM person where 1 = 0;
+--------+
|max(age)|
+--------+
|    null|
+--------+

-- Persons whose age is unknown (`NULL`) are filtered out from the result set.
SELECT * FROM person WHERE age > 0;
+--------+---+
|    name|age|
+--------+---+
|Michelle| 30|
|    Fred| 50|
|    Mike| 18|
|     Dan| 50|
|     Joe| 30|
+--------+---+

-- `IS NULL` expression is used in disjunction to select the persons
-- with unknown (`NULL`) records.
SELECT * FROM person WHERE age > 0 OR age IS NULL;
+--------+----+
|    name| age|
+--------+----+
|  Albert|null|
|Michelle|  30|
|    Fred|  50|
|    Mike|  18|
|     Dan|  50|
|   Marry|null|
|     Joe|  30|
+--------+----+

-- Person with unknown(`NULL`) ages are skipped from processing.
SELECT age, count(*) FROM person GROUP BY age HAVING max(age) > 18;
+---+--------+
|age|count(1)|
+---+--------+
| 50|       2|
| 30|       2|
+---+--------+

-- A self join case with a join condition `p1.age = p2.age AND p1.name = p2.name`.
-- The persons with unknown age (`NULL`) are filtered out by the join operator.
SELECT * FROM person p1, person p2
    WHERE p1.age = p2.age
    AND p1.name = p2.name;
+--------+---+--------+---+
|    name|age|    name|age|
+--------+---+--------+---+
|Michelle| 30|Michelle| 30|
|    Fred| 50|    Fred| 50|
|    Mike| 18|    Mike| 18|
|     Dan| 50|     Dan| 50|
|     Joe| 30|     Joe| 30|
+--------+---+--------+---+

-- The age column from both legs of join are compared using null-safe equal which
-- is why the persons with unknown age (`NULL`) are qualified by the join.
SELECT * FROM person p1, person p2
    WHERE p1.age <=> p2.age
    AND p1.name = p2.name;
+--------+----+--------+----+
|    name| age|    name| age|
+--------+----+--------+----+
|  Albert|null|  Albert|null|
|Michelle|  30|Michelle|  30|
|    Fred|  50|    Fred|  50|
|    Mike|  18|    Mike|  18|
|     Dan|  50|     Dan|  50|
|   Marry|null|   Marry|null|
|     Joe|  30|     Joe|  30|
+--------+----+--------+----+

-- `NULL` values are put in one bucket in `GROUP BY` processing.
SELECT age, count(*) FROM person GROUP BY age;
+----+--------+
| age|count(1)|
+----+--------+
|null|       2|
|  50|       2|
|  30|       2|
|  18|       1|
+----+--------+

-- All `NULL` ages are considered one distinct value in `DISTINCT` processing.
SELECT DISTINCT age FROM person;
+----+
| age|
+----+
|null|
|  50|
|  30|
|  18|
+----+

-- `NULL` values are shown at first and other values
-- are sorted in ascending way.
SELECT age, name FROM person ORDER BY age;
+----+--------+
| age|    name|
+----+--------+
|null|   Marry|
|null|  Albert|
|  18|    Mike|
|  30|Michelle|
|  30|     Joe|
|  50|    Fred|
|  50|     Dan|
+----+--------+

-- Column values other than `NULL` are sorted in ascending
-- way and `NULL` values are shown at the last.
SELECT age, name FROM person ORDER BY age NULLS LAST;
+----+--------+
| age|    name|
+----+--------+
|  18|    Mike|
|  30|Michelle|
|  30|     Joe|
|  50|     Dan|
|  50|    Fred|
|null|   Marry|
|null|  Albert|
+----+--------+

-- Columns other than `NULL` values are sorted in descending
-- and `NULL` values are shown at the last.
SELECT age, name FROM person ORDER BY age DESC NULLS LAST;
+----+--------+
| age|    name|
+----+--------+
|  50|    Fred|
|  50|     Dan|
|  30|Michelle|
|  30|     Joe|
|  18|    Mike|
|null|   Marry|
|null|  Albert|
+----+--------+

CREATE VIEW unknown_age SELECT * FROM person WHERE age IS NULL;

-- Only common rows between two legs of `INTERSECT` are in the 
-- result set. The comparison between columns of the row are done
-- in a null-safe manner.
SELECT name, age FROM person
    INTERSECT
    SELECT name, age from unknown_age;
+------+----+
|  name| age|
+------+----+
|Albert|null|
| Marry|null|
+------+----+

-- `NULL` values from two legs of the `EXCEPT` are not in output. 
-- This basically shows that the comparison happens in a null-safe manner.
SELECT age, name FROM person
    EXCEPT
    SELECT age FROM unknown_age;
+---+--------+
|age|    name|
+---+--------+
| 30|     Joe|
| 50|    Fred|
| 30|Michelle|
| 18|    Mike|
| 50|     Dan|
+---+--------+

-- Performs `UNION` operation between two sets of data. 
-- The comparison between columns of the row ae done in
-- null-safe manner.
SELECT name, age FROM person
    UNION 
    SELECT name, age FROM unknown_age;
+--------+----+
|    name| age|
+--------+----+
|  Albert|null|
|     Joe|  30|
|Michelle|  30|
|   Marry|null|
|    Fred|  50|
|    Mike|  18|
|     Dan|  50|
+--------+----+

-- Even if subquery produces rows with `NULL` values, the `EXISTS` expression
-- evaluates to `TRUE` as the subquery produces 1 row.
SELECT * FROM person WHERE EXISTS (SELECT null);
+--------+----+
|    name| age|
+--------+----+
|  Albert|null|
|Michelle|  30|
|    Fred|  50|
|    Mike|  18|
|     Dan|  50|
|   Marry|null|
|     Joe|  30|
+--------+----+

-- `NOT EXISTS` expression returns `FALSE`. It returns `TRUE` only when
-- subquery produces no rows. In this case, it returns 1 row.
SELECT * FROM person WHERE NOT EXISTS (SELECT null);
+----+---+
|name|age|
+----+---+
+----+---+

-- `NOT EXISTS` expression returns `TRUE`.
SELECT * FROM person WHERE NOT EXISTS (SELECT 1 WHERE 1 = 0);
+--------+----+
|    name| age|
+--------+----+
|  Albert|null|
|Michelle|  30|
|    Fred|  50|
|    Mike|  18|
|     Dan|  50|
|   Marry|null|
|     Joe|  30|
+--------+----+

-- The subquery has only `NULL` value in its result set. Therefore,
-- the result of `IN` predicate is UNKNOWN.
SELECT * FROM person WHERE age IN (SELECT null);
+----+---+
|name|age|
+----+---+
+----+---+

-- The subquery has `NULL` value in the result set as well as a valid 
-- value `50`. Rows with age = 50 are returned. 
SELECT * FROM person
    WHERE age IN (SELECT age FROM VALUES (50), (null) sub(age));
+----+---+
|name|age|
+----+---+
|Fred| 50|
| Dan| 50|
+----+---+

-- Since subquery has `NULL` value in the result set, the `NOT IN`
-- predicate would return UNKNOWN. Hence, no rows are
-- qualified for this query.
SELECT * FROM person
    WHERE age NOT IN (SELECT age FROM VALUES (50), (null) sub(age));
+----+---+
|name|age|
+----+---+
+----+---+