If a LEFT JOIN and an INNER JOIN return the exact same rows, which is more efficient to use?
data one; mrn='1111';cpt='aaa';output; mrn='2222';cpt='bbb';output; mrn='3333';cpt='ccc';output; mrn='4444';cpt='ddd';output; run; data two; mrn='1111';age=22;output; mrn='2222';age=44;output; mrn='3333';age=66;output; mrn='4444';age=11;output; run; proc sql noprint; create table blah1 as select a.mrn, a.cpt, b.age from one a inner join two b on a.mrn=b.mrn; create table blah2 as select a.mrn, a.cpt, b.age from one a left join two b on a.mrn=b.mrn; quit;
Use the one that makes logical sense. In this case you have a 1 to 1 match. If you always expect this use an inner match, if you can't trust this rule and always want the information from a specific table use the right/left join instead.
Thanks. I always thought INNER JOINS were not as efficient as a LEFT JOIN or a RIGHT JOIN because INNER JOINS check rows common to both tables, whereas LEFT JOINS or RIGHT JOINS just check for rows on either the left-hand or right-hand table.
I'm curious what the exceptions are to this rule, if any?
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