Query Kya Hai
Dierdre Roussin
In a database context, a query is a request for information or data made by a user and written in a specific format. The format is determined by the query language supported by that database, such as Structured Query Language. Like other query languages, SQL provides pre-defined standardized code that contains the instructions that a database can understand to generate appropriate results in response to a user's query.
A database query may be either an action query or a select query. An action query asks for additional operations on data, such as insertion, updating, deleting or other forms of data manipulation. A select query retrieves data from the database.
Primarily, queries are used to find specific data in a database by providing or filtering explicit criteria. Some query languages allow users to write and make multi-source data queries across different types of data: structured, semi-structured and unstructured.
Queries also automate data management tasks; help users summarize data; and perform some calculations or manipulations on it, such as append, delete, make a table and find total. For example, consider a relational database, which contains records or rows of information. If SQL is the query language used for this database, by using the SQL SELECT query users can choose data from a database and return it to an application. The result of the query is stored in a result table, which is called a result-set. Users can also break down the SELECT statement into other categories, such as FROM, WHERE and ORDER BY, or use the query to group and aggregate data for analysis or summarization purposes.
A query gives meaning to the lines of code used in every data selection language so the database can understand what the user wants and execute actions accordingly. The database holds data in multiple rows and columns, with the rows containing data and the columns specifying the attributes for each piece of data. By querying the database, a user can retrieve some specific data and have it displayed in a specific format. As such, the user and the database both exchange information with each other as they both "speak" the same language.
A query parameter lets users run variations of a particular query. The user is prompted to insert a field value and then use that value to create a criterion. Users can choose from a list or menu of available parameters and use them as needed.
The most popular query language is structured query language or SQL. One reason for its popularity is that SQL is specifically designed to interact with databases, even databases containing large amounts of data. So whether a user wants to access specific data in a database or perform some kind of manipulation on the data, SQL is one of the best choices available today.
Query folding describes Microsoft Power Query's ability to generate a single query statement to retrieve and transform source data. The Power Query mashup engine, part of the Microsoft Power Apps offering, attempts to accomplish query folding to enhance data retrieval or transformation efficiency whenever possible.
Power Query is a data mashup and transformation tool that uses the M Language. For data sources such as relational and non-relational databases like Active Directory, OData or Microsoft Exchange, a mashup engine "translates" queries in M Language to a language understood by the underlying data source. This language is often SQL. When complex calculations and transformations are pushed directly to the source, Power Query uses relational database engines to handle large volumes of data efficiently.
A web search query is different from a database query. While the latter refers to a request for information from a database, a web query is specifically a search engine query, which is a user's request to a search engine to generate results based on a specific user-provided topic or keyword phrase.
Thus, a web search query describes what users search for when they type a question or a word in search engines such as Bing, Google or Yahoo. Search engine queries provide information that is much different from SQL queries, as they don't require positional parameters or keywords. Instead, every search engine uses an algorithm and specific criteria -- the details of which typically are not revealed -- to search and find the most accurate results for the requests. The results are sorted based on significance and some other parameters, such as relevance, content quality, and date. Then, the list of results, known as a search engine results page, or SERP, are displayed in the web browser application.
The three types of search queries are navigational, informational and transactional. Navigational searches are intended to find a particular website. Informational searches are designed to cover a broad topic, such a comparison between iPhone and Android devices. Finally, transactional searches seek to complete a transaction, such as the purchase of a new product on an e-commerce site like Amazon.
JavaScript's querySelector() helps users find the first element that matches a particular CSS selector. To return all matching elements, developers also use the querySelectorAll() method. Whenever the selector isn't valid, the process raises a SyntaxError exception. If there are no matches, the querySelector() returns null.
Query errors occur on Facebook for several reasons. Whenever this happens, users get a message such as "Error performing query." This is usually fixed by rebooting the device, refreshing the page, logging out from the Facebook account and then logging back in, or by clearing the device's cache and cookies.
You must provide the name of the partition key attribute and a single value for that attribute. Query returns all items with that partition key value. Optionally, you can provide a sort key attribute and use a comparison operator to refine the search results.
Use the KeyConditionExpression parameter to provide a specific value for the partition key. The Query operation will return all of the items from the table or index with that partition key value. You can optionally narrow the scope of the Query operation by specifying a sort key value and a comparison operator in KeyConditionExpression. To further refine the Query results, you can optionally provide a FilterExpression. A FilterExpression determines which items within the results should be returned to you. All of the other results are discarded.
A Query operation always returns a result set. If no matching items are found, the result set will be empty. Queries that do not return results consume the minimum number of read capacity units for that type of read operation.
DynamoDB calculates the number of read capacity units consumed based on item size, not on the amount of data that is returned to an application. The number of capacity units consumed will be the same whether you request all of the attributes (the default behavior) or just some of them (using a projection expression). The number will also be the same whether or not you use a FilterExpression.
Query results are always sorted by the sort key value. If the data type of the sort key is Number, the results are returned in numeric order; otherwise, the results are returned in order of UTF-8 bytes. By default, the sort order is ascending. To reverse the order, set the ScanIndexForward parameter to false.
A single Query operation will read up to the maximum number of items set (if using the Limit parameter) or a maximum of 1 MB of data and then apply any filtering to the results using FilterExpression. If LastEvaluatedKey is present in the response, you will need to paginate the result set. For more information, see Paginating the Results in the Amazon DynamoDB Developer Guide.
FilterExpression is applied after a Query finishes, but before the results are returned. A FilterExpression cannot contain partition key or sort key attributes. You need to specify those attributes in the KeyConditionExpression.
You can query a table, a local secondary index, or a global secondary index. For a query on a table or on a local secondary index, you can set the ConsistentRead parameter to true and obtain a strongly consistent result. Global secondary indexes support eventually consistent reads only, so do not specify ConsistentRead when querying a global secondary index.
The name of this attribute conflicts with a reserved word, so it cannot be used directly in an expression. (For the complete list of reserved words, see Reserved Words in the Amazon DynamoDB Developer Guide). To work around this, you could specify the following for ExpressionAttributeNames:
Use the : (colon) character in an expression to dereference an attribute value. For example, suppose that you wanted to check whether the value of the ProductStatus attribute was one of the following:
The condition can optionally perform one of several comparison tests on a single sort key value. This allows Query to retrieve one item with a given partition key value and sort key value, or several items that have the same partition key value but different sort key values.
If you also want to provide a condition for the sort key, it must be combined using AND with the condition for the sort key. Following is an example, using the = comparison operator for the sort key:
begins_with ( sortKeyName, :sortkeyval ) - true if the sort key value begins with a particular operand. (You cannot use this function with a sort key that is of type Number.) Note that the function name begins_with is case-sensitive.
You can optionally use the ExpressionAttributeNames parameter to replace the names of the partition key and sort key with placeholder tokens. This option might be necessary if an attribute name conflicts with a DynamoDB reserved word. For example, the following KeyConditionExpression parameter causes an error because Size is a reserved word:
The maximum number of items to evaluate (not necessarily the number of matching items). If DynamoDB processes the number of items up to the limit while processing the results, it stops the operation and returns the matching values up to that point, and a key in LastEvaluatedKey to apply in a subsequent operation, so that you can pick up where you left off. Also, if the processed dataset size exceeds 1 MB before DynamoDB reaches this limit, it stops the operation and returns the matching values up to the limit, and a key in LastEvaluatedKey to apply in a subsequent operation to continue the operation. For more information, see Query and Scan in the Amazon DynamoDB Developer Guide.
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