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Is it wrong to join to another table after group by?
I don’t think that it’s “wrong” in this case, because you’ve got a one-to-one relationship between city name and city key. You could rewrite it such that you join to a sub-select to get the count of persons to cities by key, to the city table again for the name, but it’s debatable that that’d be better.
Do you have to select all columns in group by?
The point here is that it’s not that the GROUP BY has to name all the columns in the SELECT, but in fact it is the opposite – the SELECT cannot include any columns not already in the GROUP BY. Your query would only work on MySQL, because you group on Person.cityKey but select city.key.
Do you need to sort origin and destination in SQL?
You need to sort origin and destination first, to make the pairs with the same end points equal. As it’s only two columns, you can do that in a CASE
How to do a SQL pair query for travel company?
SQL Pair Query? I have only one table which contains 3 columns for travel company. It shows buses that went from City A to City B etc. I would like to find how many times this route has been used.
How does the Order of the tables affect the left join?
In contrast to the INNER JOIN, the order of the tables plays an important role in the LEFT JOIN, and the results may be completely different if the order changes in your SQL query. When determining the order of tables in a LEFT JOIN, the general rule is to start with the table from which you want to keep all the records in the final result.
How to select the last customer in SQL?
FROM customer c INNER JOIN ( SELECT customer_id, MAX (date) MaxDate FROM purchase GROUP BY customer_id ) MaxDates ON c.id = MaxDates.customer_id INNER JOIN purchase p ON MaxDates.customer_id = p.customer_id AND MaxDates.MaxDate = p.date The select should join on all customers and their Last purchase date.
When to use the left join in SQL?
The LEFT JOIN is frequently used for analytical tasks. First, it is very useful for identifying records in a given table that do not have any matching records in another. In this case, you can add a WHERE clause to the query to select, from the result of the join, the rows with NULL values in all of the columns from the second table.