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Which join is better peforming if all of them provides the same result? For example, I have a two tables employees(emp_id,name, address, designation, age, sex) and work_log(emp_id,date,hours_wored). To get some specific results both inner join and left join gives the same result. But, I have still some doubts which is not limited to this question only.

  • which join is more efficient which should prefer in case of same result values ?
  • What are other factors which must be considered at the time of applying join?
  • Is there any relationship between inner join and cross join?
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There is not a "better" or a "worse" join type. They have different meaning and they must be used depending on it.

In your case, you probably do not have employees with no work_log (no rows in that table), so LEFT JOIN and JOIN will be equivalent in results. However, if you had such a thing (a new employee with no registered work_log), a JOIN wold omit that employee, while a left join (whose first table is employees) would show all of them, and nulls on the fields from work_log if there are not matches.

Visual explanation of JOIN types

Again, performance is a secondary thing to query correctness. Some people say that you shouldn't use LEFT JOINs. It is true that a LEFT JOIN forces the optimizer to execute the query in one particular order, preventing some optimizations (table reordering) in some cases. Here is one example. But you should not choose one over the other if correctness/meaning is sacrified, as an INNER JOIN is not inherently worse. The rest of the usual optimizations apply as usual.

In summary, do not use LEFT JOIN if you really mean INNER JOIN.

In MySQL CROSS JOIN, INNER JOIN and JOIN are the same. In the standard, and semantically, a CROSS JOIN is an INNER JOIN without an ON clause, so you get every combination of rows between tables.

You have examples of all semantic types of join on Wikipedia. In practice, in MySQL, we tend to only write JOIN and LEFT JOIN.

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Which join is better performing if all of them provides the same result?

I addition of the previous answer, for what I know, MySQL is optimized to have the same performance.

With good indexes, for example, JOIN vs LEFT JOIN + WHERE clause to filter, will be the same thing. Optimization vs Human Reading take sense on large queries with lot of joins.

Using good indexes and cache is more important.

You can read a good explanation of the optimization process here :

The Query Optimization Process : A query can often be executed many different ways and produce the same result. The optimizer’s job is to find the best option.

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