Thank you for contacting Syncfusion support.
DataManager provides support for multiple queries in order to perform various operations like filtering, sorting, cloning, expanding, searching, grouping etc., in the data source. Please refer to the below help document,
The query-string is a property that has to be specified within the Grid , which defines the relation between the parent and child grid. The query-string property is used to denote the primaryKey field of the parent grid which is to be mapped with the foreignKey field of the child grid. Based on the mapping, the child grid records are filtered from the table and is bound as datasource for the child grid.