We can say that their logical working is different. I judge the logical thinking of the programmer by looking at the style of writing joins or queries in general. Also subquery returning duplicate recodes. If the code accomplishes it’s aims and is able to be maintained, then you’re doing it “right”. So, if you need to adjust the query such that limitations on either sides of the tables should be in-place, the JOIN is more preferred: SELECT * FROM A LEFT OUTER JOIN B ON A.id=B.id WHERE A.x=123; So in turn, the comma syntax will have to re-code the whole structure and adopt the join syntax instead. Read “Join Processing Changes in MySQL 5.0.12” on http://dev.mysql.com/doc/refman/5.0/en/join.html . Rob: You are missing the fact that JOIN .. ON .. syntax does *not* prevent having join conditions buried somewhere in WHERE clause in case of inner joins. Virtually any expression that would work in a WHERE clause is okay for an ON clause. By using joins, you can retrieve data from two or more tables based on logical relationships between the tables. So, to optimize performance, you need to be smart in using and selecting which one of the operators. The splitting of these purposes with their respective clauses makes the query the most readable, it also prevents incorrect data being retrieved when using JOINs types other than INNER JOIN. I want to update a table with a value that has no apparent relationship with the table containing that value (no foreign key relationship), e.g. If you have a where on the table you're outer joining to, you effectively make the query an inner join. Best to test both. As such, the employees table is likely to become the driving row source, for a filter like LIKE last_name = 'X%' is probably very selective in many instances, which means that the number of iterations will be relatively low. A typical join condition specifies a foreign key from one table and its associated key in the other table. Depending on indexes, you can get a different query plan with IN vs. an inner join (especially true in SQL Server 2000, not so much in 2005), and using nested WHERE IN might result in better performance. I prefer it myself. You can optimizing joins by choosing proper indexes, just like searching in the WHERE clause. On the other hand, when you use JOINS you might not get the same result set as in the IN and the EXISTS clauses. DISTINCT on a column marked as UNIQUE and NOT NULL is redundant, so the IN is equivalent to a simple JOIN 3. Performance Problem When Using OR In A JOIN. I thought having too much in the ON clause might slow down the query. Should the SQL engine decide on nested loops for Query 3, it is to be expected that the departments table be promoted to the position of driving row source because Oracle can use the single-column join condition on last_name as an access predicate. As such, ... Oracle will apply the filter because it knows that single-column join conditions in the ON clause of inner joins are the same as predicates in the WHERE clause. Percona's experts can maximize your application performance with our open source database support, managed services or consulting. as stated above. 2) join, id of table to be updated in ON clause, the other id in where clause. First, specify columns from both tables that you want to select data in the SELECT clause. A join clause is used to combine records or to manipulate the records from two or more tables through a join condition. 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