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
There are two formats of case expression. You can do CASE with many WHEN as; Or a Simple CASE expression Or CASE within CASE as;
Like the error says, row is a tuple, so you can’t do row[“pool_number”]. You need to use the index: row[0].
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). That … Read more
They don’t have the same structure… I can guarantee they are different I know you’ve already created it… There is already an object named ‘tbltable1’ in the database What you may want is this (which also fixes your other issue):
I have tested the following and this does work. The answer by gordyii was close but had the multiplication of 100 in the wrong place and had some missing parenthesis.
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
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:
They are speaking the same thing and it has to do with tuples (relational algebra) or rows (layman’s term). When it says high-cardinality are possible values of particular attribute (or field) that are unique and therefore the number of rows or tuples are higher: Example: As far as as StudentID the cardinality is high because it is unique. In this it … Read more