Google Sheets Lookup Row and Get Last Value - google-sheets

Question

I have a Google Sheet with two sheets in it.
The first sheet is a list of songs with Title, Library #, Composer, ..., Last Performance Date
The second sheet is a list of performance dates with Title, Library #, date 1, date 2, ..., date n
Sheet 1 Example:
Title, Library #, Composer, Last Performance Date
Hip Song 1007 David {formula}
Slow Song 1002 Bob {formula}
Other Song 1004 David {formula}
Sheet 2 Example:
Title, Library #, Dates ->
Slow Song 1002 2021-01-12 2021-02-15
Other Song 1004
Hip Song 1007 2021-01-05
How can I automatically fill in the "Last Performance Date" column on the first sheet by looking up the song on the second sheet by the "Library #" and getting the last value in that row (with a variable number of dates per song on the second sheet).
With the example above, the Last Performance Date for "Hip Song" would be "2021-01-05", "Slow Song" would be "2021-02-15", and "Other Song" would be "" (an empty string or nothing).
For any given song, there could be no dates on the second sheet or a 100+ dates on that song's row.

Answer 1

I'd add a helper column to Sheet2 that contains the formula that calculates the "latest" date for a given song:
Sheet2
A
B
C
D
E
1
Title
Library #
Latest Date
Dates
2
Slow Song
1002
=IF(MAX(D2:E2)=0,"",MAX(D2:E2))
2021-01-12
2021-02-15
3
Other Song
1004
=IF(MAX(D3:E3)=0,"",MAX(D3:E3))
4
Hip Song
1007
=IF(MAX(D4:E4)=0,"",MAX(D4:E4))
2021-01-05
The IF(X=0,"",X) part of the formula is so that songs that don't have any dates show up as blank instead of 0 (which will get formatted as 1899-12-30).
Then, in Sheet1, use the VLOOKUP function to find the matching "latest date":
Sheet1
A
B
C
D
1
Title
Library #
Composer
Last Performance Date
2
Hip Song
1007
David
=VLOOKUP(B2,Sheet2!$B$2:$C$4,2,FALSE)
3
Slow Song
1002
Bob
=VLOOKUP(B3,Sheet2!$B$2:$C$4,2,FALSE)
4
Other Song
1004
David
=VLOOKUP(B4,Sheet2!$B$2:$C$4,2,FALSE)
Note the FALSE as the last parameter to VLOOKUP, which makes sure it will still return correct results, even if the data in Sheet2 is not sorted by Library #.
See this example spreadsheet.

Answer 2

if your dates (if present) are increasing with each next column try:
=INDEX(IF(B2:B="",,IFERROR(1*SPLIT(FLATTEN(QUERY(TRANSPOSE(IFNA(VLOOKUP(B2:B, Sheet2!B1:1000,
TRANSPOSE(SORT(SEQUENCE(COLUMNS(Sheet2!C1:1))+1, 1, 0))), 0)),,9^9)), " "))),,1)
demo spreadsheet
fx transcript: count columns from column with 1st date and create a sequence of number with +1 offset for each number. then sort the sequence in descending order and transpose it into row - this will be the 3rd parameter of vlookup which will return date columns in reverse order eg.: ..., Col5, Col4, Col3, Col2. so in vlookup we look for B2:B values in range Sheet2:B1:1000 and return multiple columns on each match. next, we transpose all results of vlookup and with query parameter ,,9^9 we collapse all rows of each column into a single row. next, we flatten the results and split by empty space (which was inserted after each date via query). in this stage, column with the latest date became our 1st column. next, dates are numeric values so we multiply them by 1 to catch errors of those cells that have no date only empty spaces. at last, we use simple if statement to check if B2:B range is empty. if so we return no value, otherwise we return date. and to limit the output only to 1 single column we use index to return all rows but only the first column