How to Select Top 100 rows in Oracle?

Assuming that create_time contains the time the order was created, and you want the 100 clients with the latest orders, you can: add the create_time in your innermost query order the results of your outer query by the create_time desc add an outermost query that filters the first 100 rows using ROWNUM Query: UPDATE for Oracle 12c … Read more

Why does NULL = NULL evaluate to false in SQL server

Think of the null as “unknown” in that case (or “does not exist”). In either of those cases, you can’t say that they are equal, because you don’t know the value of either of them. So, null=null evaluates to not true (false or null, depending on your system), because you don’t know the values to … Read more

must appear in the GROUP BY clause or be used in an aggregate function

Yes, this is a common aggregation problem. Before SQL3 (1999), the selected fields must appear in the GROUP BY clause[*]. To workaround this issue, you must calculate the aggregate in a sub-query and then join it with itself to get the additional columns you’d need to show: But you may also use window functions, which looks simpler: The … Read more

MySQL: Invalid use of group function

You need to use HAVING, not WHERE. The difference is: the WHERE clause filters which rows MySQL selects. Then MySQL groups the rows together and aggregates the numbers for your COUNT function. HAVING is like WHERE, only it happens after the COUNT value has been computed, so it’ll work as you expect. Rewrite your subquery as: