解释MySQL查询

Question

Some days ago I asked here for a solution to retrieve a query which gets values from two diferent tables. The question was asked quite fast, thanks to this awesome community here... But continuing: I used the query and it worked perfectly, but I don't understand it. The query is following:

SELECT e.*, u.Username 
FROM Entries AS e 
LEFT JOIN Users AS u ON u.ID = e.User 
WHERE e.Category = 'SomeCategory' 
LIMIT 0, 10

Can anyone explain what each "step" of the query does? "WHERE","LIMIT","SELECT" and "FROM" I know, but the others get quite complicated for me.

Thanks

EDIT: LIMIT 10,0 changed to LIMIT 0,10. I written it the wrong way... Sorry about that.

Answer 1

SELECT e.*, u.Username 
FROM Entries AS e 
LEFT JOIN Users AS u ON u.ID = e.User 
WHERE e.Category = 'SomeCategory' 
LIMIT 10, 0;   -- are you sure about it???

How it works:

1. Get Entries table and "rename" it to e (add alias)
2. Get Users table and "rename" it to u
3. Join (outer) both tables based on ID column from Entries and User from Users (for non matched rows from Users set NULL)
4. Filter out records that value in Category column is equal 'SomeCategory'
5. Skip first 10 rows and get 0 rows
6. Show every column from Entries and Username column from Users

Warning

Without explicit ORDER BY you can get different resultsets between executions. Why you want to return empty resultset LIMIT offset, row_count?

EDIT:

After your edit 5. should be Skip first 0 rows and get 10 rows

LIMIT 0, 10 is the same as LIMIT 10