Here is a simple example which shows that the first two steps can generate different results:
The first PROC SQL step joins A and B on A.id=B.id to the intermediate result "one obs. with id=1, x=2, y=1" (call this T) and this with C on T.id=C.id & T.x=C.x, which results in T.
The second PROC SQL step first joins B and C on B.id=C.id to the intermediate result "one obs. with id=1, x=1, y=1" (call this V) and then joins A with V on A.id=V.id & A.x=V.x, which results in a single observation with id=1, x=2, y=. (missing), because the keys do not match.
Other examples show that even if both steps create the same observations, the results can differ in the order of observations and in the order of variables.
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