Sign Up

Sign Up to our social questions and Answers Engine to ask questions, answer people’s questions, and connect with other people.

Have an account? Sign In

Have an account? Sign In Now

Sign In

Login to our social questions & Answers Engine to ask questions answer people’s questions & connect with other people.

Sign Up Here

Forgot Password?

Don't have account, Sign Up Here

Forgot Password

Lost your password? Please enter your email address. You will receive a link and will create a new password via email.

Have an account? Sign In Now

You must login to ask question.

Forgot Password?

Need An Account, Sign Up Here

Please briefly explain why you feel this question should be reported.

Please briefly explain why you feel this answer should be reported.

Please briefly explain why you feel this user should be reported.

Sign InSign Up

StackOverflow Point

StackOverflow Point Navigation

  • Web Stories
  • Badges
  • Tags
Search
Ask A Question

Mobile menu

Close
Ask a Question
  • Web Stories
  • Badges
  • Tags
Home/ Questions/Q 3529
Alex Hales
  • 0
Alex HalesTeacher
Asked: June 2, 20222022-06-02T18:09:25+00:00 2022-06-02T18:09:25+00:00

mysql – How do I decide when to use right joins/left joins or inner joins Or how to determine which table is on which side?

  • 0

[ad_1]

I think what you’re looking for is to do a LEFT JOIN starting from the main-table to return all records from the main table regardless if they have valid data in the joined ones (as indicated by the top left 2 circles in the graphic)

JOIN’s happen in succession, so if you have 4 tables to join, and you always want all the records from your main table, you need to continue LEFT JOIN throughout, for example:

SELECT * FROM main_table
LEFT JOIN sub_table ON main_table.ID = sub_table.main_table_ID
LEFT JOIN sub_sub_table on main_table.ID = sub_sub_table.main_table_ID

If you INNER JOIN the sub_sub_table, it will immediately shrink your result set down even if you did a LEFT JOIN on the sub_table.

Remember, when doing LEFT JOIN, you need to account for NULL values being returned. Because if no record can be joined with the main_table, a LEFT JOIN forces that field to appear regardless and will contain a NULL. INNER JOIN will obviously just “throw away” the row instead because there’s no valid link between the two (no corresponding record based on the ID’s you’ve joined)

However, you mention you have a where statement that filters out the rows you’re looking for, so your question on the JOIN’s are null & void because that is not your real problem. (This is if I understand your comments correctly)

[ad_2]

  • 0 0 Answers
  • 2 Views
  • 0 Followers
  • 0
Share
  • Facebook
  • Report
Leave an answer

Leave an answer
Cancel reply

Browse

Sidebar

Ask A Question

Related Questions

  • xcode - Can you build dynamic libraries for iOS and ...

    • 0 Answers
  • bash - How to check if a process id (PID) ...

    • 8056 Answers
  • database - Oracle: Changing VARCHAR2 column to CLOB

    • 1842 Answers
  • What's the difference between HEAD, working tree and index, in ...

    • 1924 Answers
  • Amazon EC2 Free tier - how many instances can I ...

    • 0 Answers

Stats

  • Questions : 43k

Subscribe

Login

Forgot Password?

Footer

Follow

© 2022 Stackoverflow Point. All Rights Reserved.

Insert/edit link

Enter the destination URL

Or link to existing content

    No search term specified. Showing recent items. Search or use up and down arrow keys to select an item.