Table:
0
A
B
C
D
1
invoice number
order id
invoice date
invoices
2
1
9
2018-03-01 16:40:48
3
2
9
2018-03-20 00:21:41
4
3
3
2018-03-05 08:47:33
5
4
9
2018-02-28 19:01:06
6
5
13
2018-03-07 02:04:32
7
6
8
2018-03-16 21:57:43
8
7
10
2018-03-13 07:27:38
9
8
10
2018-03-19 17:06:30
10
9
12
2018-03-16 11:01:06
11
10
11
2018-03-01 01:44:08
12
11
5
2018-03-23 04:59:28
13
12
3
2018-03-15 21:24:13
14
13
3
2018-03-03 20:44:06
15
14
4
2018-03-19 22:38:10
16
15
14
2018-03-15 09:38:49

Questions and formulas:
Question: How many invoices do we have?
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D16))
Level: easy
Question: Count the number of invoices.
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D16))
Level: easy
Question: Show the order ids and the number of invoices for each order id.
Formula: SUMMARIZE(B1,DCOUNTX(D1))
Formula2: SUMMARIZE(B2:B16,DCOUNTX(D2:D16))
Level: easy
Question: How many invoices correspond to each order id?
Formula: SUMMARIZE(B1,DCOUNTX(D1))
Formula2: SUMMARIZE(B2:B16,DCOUNTX(D2:D16))
Level: easy
Question: How many invoices on record?
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D16))
Level: easy
Question: The number of invoices.
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D16))
Level: easy
Question: The number of invoices for each order.
Formula: SUMMARIZE(B1,DCOUNTX(D1))
Formula2: SUMMARIZE(B2:B16,DCOUNTX(D2:D16))
Level: easy
Question: the number of invoices for each order
Formula: SUMMARIZE(B1,DCOUNTX(D1))
Formula2: SUMMARIZE(B2:B16,DCOUNTX(D2:D16))
Level: easy
Question: How many invoices?
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D16))
Level: easy