Table:
0
A
B
C
D
1
author id
last name
first name
authors
2
50
Gibbons
Jeremy
3
51
Hinze
Ralf
4
52
James
Daniel W. H.
5
53
Shivers
Olin
6
54
Turon
Aaron
7
55
Ahmed
Amal
8
56
Blume
Matthias
9
57
Ohori
Atsushi
10
58
Ueno
Katsuhiro
11
59
Pouillard
Nicolas
12
60
Weirich
Stephanie
13
61
Yorgey
Brent
14
62
Sheard
Tim

Questions and formulas:
Question: How many authors are there?
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D14))
Level: easy
Question: Count the number of authors.
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D14))
Level: easy
Question: What is the first name of the author with last name "Ueno"?
Formula: LET(query1,FILTER(A1:D1,B1="ueno"),UNIQUE(CHOOSECOLS(query1,3,4)))
Formula2: LET(query1,FILTER(A2:D14,B2:B14="ueno"),UNIQUE(CHOOSECOLS(query1,3,4)))
Level: hard
Question: Which authors have last name "Ueno"? List their first names.
Formula: LET(query1,FILTER(A1:D1,B1="ueno"),UNIQUE(CHOOSECOLS(query1,4,3)))
Formula2: LET(query1,FILTER(A2:D14,B2:B14="ueno"),UNIQUE(CHOOSECOLS(query1,4,3)))
Level: hard
Question: Find the last name of the author with first name "Amal".
Formula: LET(query1,FILTER(A1:D1,C1="amal"),UNIQUE(CHOOSECOLS(query1,2,4)))
Formula2: LET(query1,FILTER(A2:D14,C2:C14="amal"),UNIQUE(CHOOSECOLS(query1,2,4)))
Level: hard
Question: Which authors have first name "Amal"? List their last names.
Formula: LET(query1,FILTER(A1:D1,C1="amal"),UNIQUE(CHOOSECOLS(query1,4,2)))
Formula2: LET(query1,FILTER(A2:D14,C2:C14="amal"),UNIQUE(CHOOSECOLS(query1,4,2)))
Level: hard
Question: first name of author with last name ueno
Formula: LET(query1,FILTER(A1:D1,B1="ueno"),UNIQUE(CHOOSECOLS(query1,3,4)))
Formula2: LET(query1,FILTER(A2:D14,B2:B14="ueno"),UNIQUE(CHOOSECOLS(query1,3,4)))
Level: hard
Question: What is ueno's first name?
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:D1,B1="ueno"),3))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:D14,B2:B14="ueno"),3))
Level: medium
Question: What is amal's last name?
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:D1,C1="amal"),2))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:D14,C2:C14="amal"),2))
Level: medium
Question: last name of author with first name amal 
Formula: LET(query1,FILTER(A1:D1,C1="amal"),UNIQUE(CHOOSECOLS(query1,2,4)))
Formula2: LET(query1,FILTER(A2:D14,C2:C14="amal"),UNIQUE(CHOOSECOLS(query1,2,4)))
Level: hard
Question: total number of authors
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D14))
Level: easy
Question: How many authors are there?
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D14))
Level: easy
Question: the number of authors
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D14))
Level: easy
Question: How many authors?
Formula: ROWS(UNIQUE(D1))
Formula2: ROWS(UNIQUE(D2:D14))
Level: easy
Question: First name of the author whose last name is ueno
Formula: LET(a,HSTACK(C1,D1),b,D1,c,CHOOSECOLS(UNIQUE(CHOOSECOLS(FILTER(A1:D1,B1="ueno"),4)),1),UNIQUE(FILTER(a,NOT(ISNA(XLOOKUP(b,c,c))))))
Formula2: LET(a,HSTACK(C2:C14,D2:D14),b,D2:D14,c,CHOOSECOLS(UNIQUE(CHOOSECOLS(FILTER(A2:D14,B2:B14="ueno"),4)),1),UNIQUE(FILTER(a,NOT(ISNA(XLOOKUP(b,c,c))))))
Level: hard
Question: First name of the author whose last name is ueno
Formula: LET(a,HSTACK(C1,D1),b,D1,c,CHOOSECOLS(UNIQUE(CHOOSECOLS(FILTER(A1:D1,B1="ueno"),4)),1),UNIQUE(FILTER(a,NOT(ISNA(XLOOKUP(b,c,c))))))
Formula2: LET(a,HSTACK(C2:C14,D2:D14),b,D2:D14,c,CHOOSECOLS(UNIQUE(CHOOSECOLS(FILTER(A2:D14,B2:B14="ueno"),4)),1),UNIQUE(FILTER(a,NOT(ISNA(XLOOKUP(b,c,c))))))
Level: hard
Question: the last name of the author whose first name is amal.
Formula: LET(a,HSTACK(B1,D1),b,D1,c,CHOOSECOLS(UNIQUE(CHOOSECOLS(FILTER(A1:D1,C1="amal"),4)),1),UNIQUE(FILTER(a,NOT(ISNA(XLOOKUP(b,c,c))))))
Formula2: LET(a,HSTACK(B2:B14,D2:D14),b,D2:D14,c,CHOOSECOLS(UNIQUE(CHOOSECOLS(FILTER(A2:D14,C2:C14="amal"),4)),1),UNIQUE(FILTER(a,NOT(ISNA(XLOOKUP(b,c,c))))))
Level: hard
Question: Last name of the author whose first name is amal 
Formula: LET(a,HSTACK(B1,D1),b,D1,c,CHOOSECOLS(UNIQUE(CHOOSECOLS(FILTER(A1:D1,C1="amal"),4)),1),UNIQUE(FILTER(a,NOT(ISNA(XLOOKUP(b,c,c))))))
Formula2: LET(a,HSTACK(B2:B14,D2:D14),b,D2:D14,c,CHOOSECOLS(UNIQUE(CHOOSECOLS(FILTER(A2:D14,C2:C14="amal"),4)),1),UNIQUE(FILTER(a,NOT(ISNA(XLOOKUP(b,c,c))))))
Level: hard