如何对MySQL JSON列属性进行空检查？

如何解决如何对MySQL JSON列属性进行空检查？

我正在使用MySQL 8.0.21。我需要编写一个与JSON列类型一起使用的查询。 JSON文档中的某些数据具有空值，我想过滤掉这些空值。

示例中可能存在的行，为简单起见，已删除了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;

我注意到了一些有趣的事情...

比较其他值起作用。例如...

-- 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;

在上面的一些示例的注释中提到的另一件有趣的事情是对null检查的奇怪行为。 如果jsonColumn为null，即使知道我正在访问jsonColumn->'$。value'，null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，将->>以及JSON_UNQUOTE和JSON_EXTRACT与IS NOT NULL比较，应该可以正常工作。我以为那是可以的。

老实说，这可能是IS语句和JSON列类型的错误。将其与JSON文档而不是JSON文档的值进行比较时已经存在一些奇怪的行为。

无论如何，有什么方法可以做到这一点？还是我一直在尝试的方法被证实是正确的方法，而这仅仅是一个错误？

解决方法

在Barmar发表评论之后...

显然，此更改在8.0.13之前的某个时候发生了。 forums.mysql.com/read.php?176,670072

论坛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;

论坛帖子上说（关于变通方法之前发布的代码）...

C2实际上设置为NULL，但是C1作为4个字符的“ 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.