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How to find the Query Builder in the freshly released QGIS version 3.0?
In previous versions (2.x), it could be found as mentioned in Displaying only selected features on map in QGIS?
In QGIS3, the query builder button is located in the Layer Properties dialog, then in the Source tab, and finally in the bottom right corner of Provider feature filter.
Provider feature filter
As shown in the below picture, you find the Query Builder by opening the Layer Properties and going to 'Source' menu. Under Feature Filter, click on the [Query Builder] button to open the Query Builder.
Required, but never shown