A SQL Knowledge Graph is the implementation of ontologies and graph theory in standard SQL. It has three components: (i) a virtual SQL ontology of connected, context-enriched concepts with inference capabilities and graph analytics features; (ii) a mapping of the virtual SQL ontologies to existing databases accessible in SQL and, (iii) a query execution engine that translates SQL queries of the ontology into SQL queries pushed down to the underlying databases. The SQL Knowledge Graph closes the gap between knowledge representation and enterprise databases/legacy systems/data warehouses/data lakes, to conveniently enable smart, semantic data fabrics and digital twins without need to change DBMS infrastructure.