In what order are MySQL JOINs evaluated?

by

  1. USING (fieldname) is a shorthand way of saying ON table1.fieldname = table2.fieldname.

  2. SQL doesn’t define the ‘order’ in which JOINS are done because it is not the nature of the language. Obviously an order has to be specified in the statement, but an INNER JOIN can be considered commutative: you can list them in any order and you will get the same results.

    That said, when constructing a SELECT … JOIN, particularly one that includes LEFT JOINs, I’ve found it makes sense to regard the third JOIN as joining the new table to the results of the first JOIN, the fourth JOIN as joining the results of the second JOIN, and so on.

    More rarely, the specified order can influence the behaviour of the query optimizer, due to the way it influences the heuristics.

  3. No. The way the query is assembled, it requires that companies and users both have a companyid, jobs has a userid and a jobid and useraccounts has a userid. However, only one of companies or user needs a userid for the JOIN to work.

  4. The WHERE clause is filtering the whole result — i.e. all JOINed columns — using a column provided by the jobs table.

More Related Contents:

  1. What’s the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN? [duplicate]
  2. 1052: Column ‘id’ in field list is ambiguous
  3. Join tables with SUM issue in MYSQL
  4. Join between tables in two different databases?
  5. Required to join 2 tables with their FKs in a 3rd table
  6. How can I join multiple SQL tables using the IDs?
  7. Why does MySQL report a syntax error on FULL OUTER JOIN?
  8. Mixing ANSI 1992 JOINs and COMMAs in a query
  9. Cumulative sum over a set of rows in mysql
  10. What’s the difference between comma separated joins and join on syntax in MySQL? [duplicate]
  11. MySQL JOIN the most recent row only?
  12. How to find non-existing data from another Table by JOIN?
  13. MySQL Inner Join Query Multiple Tables
  14. Difference between these two joining table approaches?
  15. Laravel join with 3 Tables
  16. MySQL LEFT JOIN 3 tables
  17. how to use a like with a join in sql?
  18. When to use a left outer join?
  19. How to store a one to many relation in MySQL database?
  20. MySQL Multiple Joins in one query?
  21. INSERT INTO SELECT statement with more columns which is not in another table
  22. ‘IF’ in ‘SELECT’ statement – choose output value based on column values
  23. JOIN queries vs multiple queries
  24. Why does MySQL allow “group by” queries WITHOUT aggregate functions?
  25. Add Auto-Increment ID to existing table?
  26. Using group by on two fields and count in SQL
  27. Can MySQL convert a stored UTC time to local timezone?
  28. MySQL CONCAT returns NULL if any field contain NULL
  29. Alternative to using LIMIT keyword in a SubQuery in MYSQL
  30. MySQL optimizing INSERT speed being slowed down because of indices

Leave a Comment