## sequence of multiple left join

Hi all,

I would like to use left join to join multiple tables and I am wondering whether the order of the join matters. Say

```proc sql;
create table t1 as
select * from A
natural left join B
natural left join C;
quit;```

does this code generate the same result as

```proc sql;
create table t1 as
select * from A
natural left join
(select * from B natural left join C);
quit;```

?

Or as long as the tables A, B, and C is linked by left join in this order, the result would not change?

I tried the above in a small trial dataset, and I got the same table. However, when I used similar multiple natural left join with the data I worked on, I got different result.

More generally, is the following correct: A left join B left join c =  (A left join B) left join c =  A left join (B left join c) =  A left join D where D = B left join c?

Thanks

## Re: sequence of multiple left join

Hi @bochen,

Here is a simple example which shows that the first two steps can generate different results:

``````data a;
id=1; x=2;
run;

data b;
id=1; y=1;
run;

data c;
id=1; x=1;
run;``````

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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