Exists query is also called NULL query or NULL-value query. This query is used in sparse data to determine whether a column of a row exists. For example, you can query the rows in which the value of the address column is not empty.
If you want to perform an exists query on a Nested column, you can use nested query.
If you want to check whether a column contains empty values, you must use ExistsQuery together with mustNotQueries of BoolQuery.
If one of the following conditions is met, the system considers that a column does not exist. In this example, the city column is used.
The type of the city column in the search index is a basic type such as keyword. If a row in which the city column does not exist in the data table, the search index considers that the city column does not exist.
The type of the city column in the search index is a basic type such as keyword. If a row in which the value of the city column is an empty array in the data table ("city" = "[]"), the search index considers that the city column does not exist.
Prerequisites
- An OTSClient instance is initialized. For more information, see Initialize an OTSClient instance.
- A data table is created. Data is written to the table.
- A search index is created for the data table. For more information, see Create search indexes.
Parameters
Parameter | Description |
TableName | The name of the data table. |
IndexName | The name of the search index. |
Query | The query type. Set the parameter to existsQuery. |
GetTotalCount | Specifies whether to return the total number of rows that meet the query conditions. The default value of this parameter is false, which specifies that the total number of rows that meet the query conditions is not returned. If you set this parameter to true, the query performance is compromised. |
FieldName | The name of the field that you want to query. |
ColumnsToGet | Specifies whether to return all columns of each row that meets the query conditions. You can configure ReturnAll, Columns, and ReturnAllFromIndex for this parameter. The default value of ReturnAll is false, which indicates that not all columns are returned. You can use one of the following methods to specify the columns that you want to return. If you do not use the following methods to specify the columns that you want to return, only the primary key columns are returned.
If you set ReturnAll to true, all columns are returned. |
Examples
The following sample code provides an example on how to query the rows in which the specified column is not empty:
/// <summary>
/// Perform an exists query to query the rows in which the value of the pk0 column is not empty.
/// </summary>
/// <param name="otsClient"></param>
public static void ExistsQuery(OTSClient otsClient)
{
ExistsQuery existsQuery = new ExistsQuery
{
FieldName = "pk0"
};
SearchQuery searchQuery = new SearchQuery
{
Query = existsQuery,
GetTotalCount = false
};
SearchRequest searchRequest = new SearchRequest(TableName, IndexName, searchQuery);
SearchResponse searchResponse = otsClient.Search(searchRequest);
Console.WriteLine(JsonConvert.SerializeObject(searchResponse));
}FAQ
References
When you use a search index to query data, you can use the following query methods: term query, terms query, match all query, match query, match phrase query, prefix query, range query, wildcard query, Boolean query, geo query, nested query, and exists query. You can use different query methods to query data from multiple dimensions based on your business requirements.
You can sort or paginate rows that meet the query conditions by using the sorting and paging features. For more information, see Sorting and paging.
You can use the collapse (distinct) feature to collapse the result set based on a specific column. This way, data of the specified type appears only once in the query results. For more information, see Collapse (distinct).
If you want to analyze data in a data table, you can use the aggregation feature of the Search operation or execute SQL statements. For example, you can obtain the minimum and maximum values, sum, and total number of rows. For more information, see Aggregation and SQL query.
If you want to obtain all rows that meet the query conditions without the need to sort the rows, you can call the ParallelScan and ComputeSplits operations to use the parallel scan feature. For more information, see Parallel scan.