Different or not? the two left join queries

cy163 · #1 (**permalink**) 08-15-09, 07:59

Hello Everybody,

I am not sure whether the following two queries are the same or not?

Code:

     SELECT 
              a.* 
     FROM  
              a 
    LEFT JOIN b

              ON   a.ID = b.ID
   
    LEFT JOIN  c
             ON  a.ID = c.ID

Code:

     SELECT 
              a.* 
     FROM  
              a 
    LEFT JOIN b

              ON   a.ID = b.ID
   
    LEFT JOIN  c
             ON  b.ID = c.ID

Thanks

r937 · #2 (**permalink**) 08-15-09, 09:07

they are not the same joins, but the results are the same

here is some test data that you can use to verify that the joins are different --

Code:

   a      b      c
  21     21     21
  22            22

however, since you are returning only the a columns, the results look the same

note also that if we change the test data as follows--

Code:

   a      b      c
  21     21     21
  22            22
                22
                22

then the results will be different

this example once again shows the folly of "dumbing down" your real question

Internet Services

Web Development

Digital Marketing

Consumer Tech