node.js 如何在 DynamoDB 中查询不存在（空）的属性
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How do you query for a non-existent (null) attribute in DynamoDB
提问by Jordan Mack
I'm trying to query a DynamoDB table to find all items where the emailattribute is not set. A global secondary index called EmailPasswordIndexexists on the table which includes the emailfield.
我正在尝试查询 DynamoDB 表以查找email未设置属性的所有项目。EmailPasswordIndex包含该email字段的表上存在一个名为的全局二级索引。
var params = {
"TableName": "Accounts",
"IndexName": "EmailPasswordIndex",
"KeyConditionExpression": "email = NULL",
};
dynamodb.query(params, function(err, data) {
if (err)
console.log(JSON.stringify(err, null, 2));
else
console.log(JSON.stringify(data, null, 2));
});
Result:
结果：
{
"message": "Invalid KeyConditionExpression: Attribute name is a reserved keyword; reserved keyword: NULL",
"code": "ValidationException",
"time": "2015-12-18T05:33:00.356Z",
"statusCode": 400,
"retryable": false
}
Table definition:
表定义：
var params = {
"TableName": "Accounts",
"KeySchema": [
{ "AttributeName": "id", KeyType: "HASH" }, // Randomly generated UUID
],
"AttributeDefinitions": [
{ "AttributeName": "id", AttributeType: "S" },
{ "AttributeName": "email", AttributeType: "S" }, // User e-mail.
{ "AttributeName": "password", AttributeType: "S" }, // Hashed password.
],
"GlobalSecondaryIndexes": [
{
"IndexName": "EmailPasswordIndex",
"ProvisionedThroughput": {
"ReadCapacityUnits": 1,
"WriteCapacityUnits": 1
},
"KeySchema": [
{ "AttributeName": "email", KeyType: "HASH" },
{ "AttributeName": "password", KeyType: "RANGE" },
],
"Projection": { "ProjectionType": "ALL" }
},
],
ProvisionedThroughput: {
ReadCapacityUnits: 1,
WriteCapacityUnits: 1
}
};
dynamodb.createTable(params, function(err, data) {
if (err)
console.log(JSON.stringify(err, null, 2));
else
console.log(JSON.stringify(data, null, 2));
});
回答by JaredHatfield
DynamoDB's Global Secondary Indexes allow for the indexes to be sparse. That means that if you have a GSI whose hash or range key for an item is not defined then that item will simply not be included in the GSI. This is useful in a number of use cases as it allows you to directly identify records that contain certain fields. However, this approach will not work if you are looking for the lack of a field.
DynamoDB 的全局二级索引允许索引稀疏。这意味着，如果您有一个 GSI，其项目的哈希或范围键未定义，那么该项目将不会包含在 GSI 中。这在许多用例中很有用，因为它允许您直接识别包含某些字段的记录。但是，如果您正在寻找缺少字段，这种方法将不起作用。
To get all of the items that have a field not set your best bet may be resorting to a scan with a filter. This operation will be very expensive but it would be straightforward code looking something like the following:
要获取具有未设置字段的所有项目，您最好的选择可能是使用过滤器进行扫描。此操作将非常昂贵，但它是简单的代码，如下所示：
var params = {
TableName: "Accounts",
FilterExpression: "attribute_not_exists(email)"
};
dynamodb.scan(params, {
if (err)
console.log(JSON.stringify(err, null, 2));
else
console.log(JSON.stringify(data, null, 2));
});
回答by Mardok
@jaredHatfield is correct if the field does not exist but that will not work if the filed is null. NULL is a keyword and can't used directly. But you can use it with ExpressionAttributeValues.
如果该字段不存在，@jaredHatfield 是正确的，但如果该字段为空，则该字段无效。NULL 是关键字，不能直接使用。但是您可以将它与 ExpressionAttributeValues 一起使用。
const params = {
TableName: "Accounts",
FilterExpression: "attribute_not_exists(email) or email = :null",
ExpressionAttributeValues: {
':null': null
}
}
dynamodb.scan(params, (err, data) => {
if (err)
console.log(JSON.stringify(err, null, 2));
else
console.log(JSON.stringify(data, null, 2));
})
