Good morning. The resources say that "left joins can deliver a list with all records from the left table that do not match any rows from the right table". So, does t1 LEFT JOIN t2 show the records of t1 that are not in t2? And if so, if t1 has 11 rows and t2 has 26 rows, shouldn't t1 LEFT JOIN t2 have at maximum 11 rows? Instead, if t1 LEFT JOIN t2 shows the records in t1 that are not in t2 + the records they share, isn't this like showing all table t1? Thank you

Hello Alessandro, The result of t1 left join t2 would the the sum of the records found in each image. In terms of the resulting table in the lecture, the records below fall within the second image. These are records from t1 that do not match any rows from t2. And here are the records where there is a (one-to-many) match in m.dept_no = d.dept_no. When it comes to the maximum number of rows, I am not certain if it is possible to reach r1 * r2 records with left join, as that number of records are attainable with Cross join only (as far as I know). Hope this helps.

Hi Alessandro! Thanks for reaching out. Yes, your logic is correct. When using LEFT JOIN the order of the tables matters (when using INNER JOIN=JOIN the order does not matter!). So, every record, that is contained in the left table, (the first one) and is not contained in the right table, (the second one) will be included in the result set. Hope this helps. Best, Tsvetelin

So if t1 has 11 rows and t2 has 26 rows, shouldn't t1 LEFT JOIN t2 have at maximum 11 rows?

Hi Alessandro! Thanks for reaching out. The result set will contain 11 rows only if these rows are not contained in the t2 table. If some of these rows are contained in the t2 table, then the result set will contain 11-n rows where n is the number of rows contained in both tables. Hope this helps. Best, Tsvetelin

Good morning. It was exactly how I thought it worked, so why the following code returns 26 rows and not at maximum 11? SELECT d.dept_no, m.emp_no, d.dept_name FROM departments_dup d LEFT JOIN dept_manager_dup m ON m.dept_no = d.dept_no ORDER BY m.dept_no; departments_dup contains only 11 rows; dept_manager_dup contains 26 rows. If it works as you said, I expect at maximum 11 rows, but it gives me 24 rows. Why? Thanks

Ok thank you. So I think that if t1 has r1 rows and t2 has r2 rows, the maximum number of rows of t1 left join t2 is r1 * r2, isn't it? But I can't understand one thing: the resources say that "left joins can deliver a list with all records from the left table that do not match any rows from the right table". But you said that we must add either this (second image) or the common rows of the two tables (first image). So what of the two things is the result of t1 left join t2? First image + second image or just second image? Thanks

Ok so the resources are just confusing with the statement "left joins can deliver a list with all records from the left table that do not match any rows from the right table", it's simply incomplete. Thank you very much!