which query is better and efficient - mysql

Question

I came across writing the query in differnt ways like shown below Type-I

SELECT JS.JobseekerID
         , JS.FirstName
         , JS.LastName
         , JS.Currency
         , JS.AccountRegDate
         , JS.LastUpdated
         , JS.NoticePeriod
         , JS.Availability
         , C.CountryName
         , S.SalaryAmount
         , DD.DisciplineName
         , DT.DegreeLevel 
    FROM Jobseekers JS 
INNER 
   JOIN Countries C 
      ON JS.CountryID = C.CountryID 
INNER 
   JOIN SalaryBracket S 
      ON JS.MinSalaryID = S.SalaryID 
INNER 
  JOIN DegreeDisciplines DD 
     ON JS.DegreeDisciplineID = DD.DisciplineID 
INNER 
  JOIN DegreeType DT 
     ON JS.DegreeTypeID = DT.DegreeTypeID 
WHERE
  JS.ShowCV = 'Yes'

Type-II

SELECT JS.JobseekerID
         , JS.FirstName
         , JS.LastName
         , JS.Currency
         , JS.AccountRegDate
         , JS.LastUpdated
         , JS.NoticePeriod
         , JS.Availability
         , C.CountryName
         , S.SalaryAmount
         , DD.DisciplineName
         , DT.DegreeLevel 
    FROM Jobseekers JS, Countries C, SalaryBracket S, DegreeDisciplines DD
         , DegreeType DT
    WHERE
           JS.CountryID = C.CountryID 
           AND JS.MinSalaryID = S.SalaryID 
           AND JS.DegreeDisciplineID = DD.DisciplineID 
           AND JS.DegreeTypeID = DT.DegreeTypeID 
           AND  JS.ShowCV = 'Yes'

I am using Mysql database

Both works really well, But I am wondering

1. which is best practice to use all time for any situation?
2. Performance wise which is better one?(Say the database as a millions records)
3. Any advantages of one over the other?
4. Is there any tool where I can check which is better query?

Thanks in advance

Answer 1

in the most code i've seen, those querys are done like your Type-II - but i think Type-I is better because of readability (and more logic - a join is a join, so you should write it as a join (althoug the second one is just another writing style for inner joins)).

in performance, there shouldn't be a difference (if there is one, i think the Type-I would be a bit faster).

Answer 2

Look at "Explain"-syntax http://dev.mysql.com/doc/refman/5.1/en/explain.html

Answer 3

My suggestion.

Update all your tables with some amount of records. Access the MySQL console and run SQL both command one by one. You can see the time execution time in the console.

Answer 4

1- It's a no brainer, use the Type I

2- The type II join are also called 'implicit join', whereas the type I are called 'explicit join'. With modern DBMS, you will not have any performance problem with normal query. But I think with some big complex multi join query, the DBMS could have issue with the implicit join. Using explicit join only could improve your explain plan, so faster result !

3- So performance could be an issue, but most important maybe, the readability is improve for further maintenance. Explicit join explain exactly what you want to join on what field, whereas implicit join doesn't show if you make a join or a filter. The Where clause is for filter, not for join !

And a big big point for explicit join : outer join are really annoying with implicit join. It is so hard to read when you want multiple join with outer join that explicit join are THE solution.

4- Execution plan are what you need (See the doc)

Some duplicates Here, here and here.

Answer 5

edit: Scorpi0 mentions it quite well, be sure to take a look at his SO-links

Answer 6

For the two queries you mentioned (each with only inner joins) any modern database's query optimizer should produce exactly the same query plan, and thus the same performance.

For MySQL, if you prefix the query with EXPLAIN, it will spit out information about the query plan (instead of running the query). If the information from both queries is the same, them the query plan is the same, and the performance will be identical. From the MySQL Reference Manual:

EXPLAIN returns a row of information for each table used in the SELECT statement. The tables are listed in the output in the order that MySQL would read them while processing the query. MySQL resolves all joins using a nested-loop join method. This means that MySQL reads a row from the first table, and then finds a matching row in the second table, the third table, and so on. When all tables are processed, MySQL outputs the selected columns and backtracks through the table list until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table.

When the EXTENDED keyword is used, EXPLAIN produces extra information that can be viewed by issuing a SHOW WARNINGS statement following the EXPLAIN statement. This information displays how the optimizer qualifies table and column names in the SELECT statement, what the SELECT looks like after the application of rewriting and optimization rules, and possibly other notes about the optimization process.

As to which syntax is better? That's up to you, but once you move beyond inner joins to outer joins, you'll need to use the newer syntax, since there's no standard for describing outer joins using the older implicit join syntax.