如何对 MySQL JSON 列属性进行空检查？

Question

我正在使用 MySQL 8.0.21。我需要编写一个适用于 JSON 列类型的查询。 JSON 文档中的一些数据有空值，我想过滤掉这些空值。

为简单起见，删除了可能行的示例，JSON 文档中的大多数属性：

jsonColumn
'{"value":96.0}'
'{"value":null}' -- This is the row I am trying to filter out
NULL

这是我尝试过的：

-- Removed columns where jsonColumn was NULL but, NOT columns where jsonColumn->'$.value' was null. 
SELECT * 
FROM <table>
WHERE jsonColumn->'$.value' IS NOT NULL;

-- Note the unquote syntax, ->>. The code above uses ->.
-- Produced the same result as the code above.
SELECT * 
FROM <table>
WHERE jsonColumn->>'$.value' IS NOT NULL;

-- Produced same result as the two above. Not surprised because -> is an alias of JSON_EXTRACT
SELECT * 
FROM <table>
WHERE JSON_EXTRACT(jsonColumn, '$.value') IS NOT NULL;

-- Produced same result as the three above. Not surprised because ->> is an alias of JSON_EXTRACT
SELECT * 
FROM <table>
WHERE JSON_UNQUOTE(JSON_EXTRACT(jsonColumn, '$.value')) IS NOT NULL;

-- Didn't really expect this to work. It didn't work. For some reason it filters out all records from the select.
SELECT *
FROM <table>
WHERE jsonColumn->'$.value' != NULL;

-- Unquote syntax again. Produced the same result as the code above.
SELECT *
FROM <table>
WHERE jsonColumn->>'$.value' != NULL;

-- Didn't expect this to work. Filters out all records from the select.
SELECT *
FROM <table>
WHERE JSON_EXTRACT(jsonColumn, '$.value') != NULL;

-- Didn't expect this to work. Filters out all records from the select.
SELECT *
FROM <table>
WHERE JSON_UNQUOTE(JSON_EXTRACT(jsonColumn, '$.value')) != NULL;

-- I also tried adding a boolean value to one of the JSON documents, '{"test":true}'. These queries did not select the record with this JSON document.
SELECT * 
FROM <table>
WHERE jsonColumn->'$.test' IS TRUE;
SELECT * 
FROM <table>
WHERE jsonColumn->>'$.test' IS TRUE;

我注意到一些有趣的事情......

比较其他值有效。比如……

-- This query seems to work fine. It filters out all records except those where jsonColumn.value is 96.
SELECT *
FROM <table>
WHERE jsonColumn->'$.value' = 96;

我注意到的另一件有趣的事情，在上面的一些示例的 cmets 中提到，是 null 检查的一些奇怪行为。 如果 jsonColumn 为空，即使知道我正在访问 jsonColumn->'$.value'，空检查也会过滤掉记录。

不确定这是否清楚，所以让我详细说明一下......

-- WHERE jsonColumn->>'$.value' IS NOT NULL
jsonColumn
'{"value":96.0}'
'{"value":null}' -- This is the row I am trying to filter out. It does NOT get filtered out.
NULL -- This row does get filtered out.

根据this post，使用 ->> 和 JSON_UNQUOTE 和 JSON_EXTRACT 与 IS NOT NULL 比较应该有效。我认为它在当时有效。

老实说，这可能是 IS 语句和 JSON 列类型的错误。在比较 JSON 文档而不是 JSON 文档的值时，已经存在奇怪的行为。

无论如何，有没有办法做到这一点？还是我一直在尝试的方法被证实是正确的方法，而这只是一个错误？

Answer 1

根据 Barmar 的评论...

显然，这在 8.0.13 之前的某个时候发生了变化。 forums.mysql.com/read.php?176,670072,670072

论坛帖子中的解决方法似乎使用 JSON_TYPE。看起来是一个糟糕的解决方法。

SET @doc = JSON_OBJECT('a', NULL);
SELECT JSON_UNQUOTE(IF(JSON_TYPE(JSON_EXTRACT(@doc,'$.a')) = 'NULL', NULL, JSON_EXTRACT(@doc,'$.a'))) as C1,
JSON_UNQUOTE(JSON_EXTRACT(@doc,'$.b')) as C2;

论坛帖子说（关于解决方法之前发布的代码）...

C2 被有效地设置为 NULL，但 C1 作为 4 char 'null' 字符串返回。

所以我开始搞乱字符串比较...

// This filtered out NULL jsonColumn but, NOT NULL jsonColumn->'$.value'
SELECT *
FROM <table>
WHERE jsonColumn->'$.value' != 'null';

jsonColumn
'{"value":96.0}'
'{"value":"null"}' -- Not originally apart of my dataset but, this does get filtered out. Which is very interesting...
'{"value":null}' -- This does NOT get filtered out.
NULL -- This row does get filtered out.

// This filtered out both NULL jsonColumn AND NULL jsonColumn->'$.value'
SELECT *
FROM <table>
WHERE jsonColumn->>'$.value' != 'null';

jsonColumn
'{"value":96.0}'
'{"value":"null"}' -- Not originally apart of my dataset but, this does get filtered out.
'{"value":null}' -- This does get filtered out.
NULL -- This row does get filtered out.