You cannot turn ALL queries with subqueries into JOINs.
Example: correlated subquery
FROM Production.Product p1
INNER JOIN Production.ProductSubcategory sc
ON sc.ProductSubcategoryID = p1.ProductSubcategoryID
WHERE p1.ListPrice >
(SELECT AVG (p2.ListPrice)
FROM Production.Product p2
WHERE p1.ProductSubcategoryID = p2.ProductSubcategoryID)
ORDER BY Subcategory, AboveAvgListPrice
However, you can turn some into JOINS when subquery was really not forced.
SQL Server query engine turns the query into an actual program based on optimization algorithms. The execution plan includes several steps and intermediate (temp) tables.
Kalman Toth, Business Intelligence Architect