CROSS JOIN UNNEST in Presto
CROSS JOIN UNNEST in Presto
What is Presto and what is the meaning of un-nesting an array?
As the data grows, it becomes trickier to hold
CROSS JOIN UNNEST in Presto

CROSS JOIN UNNEST – It is simply used to flatten an array, flattening means converting an Array, Map or Row in a flat relation by converting it into multiple rows (one row for every value in array)
Sample table
Name | Emp_id | Subject(Array) | Phone |
X | 123 | [C,JAVA,SQL] | [123,456] |
Y | 4231 | [Hive,Presto] | [542.654] |
Z | 322 | [Ruby,Perl] | [12343] |
Q | 421 | [Python,R] | [765,987] |
Presto
Select Name,Emp_id,expertise
from Employee
CROSS JOIN UNNEST(Subject) as t(expertise)
If there are multiple arrays
Select Name,Emp_id,expertise,phone_num
from Employee
CROSS JOIN UNNEST(Subject,Phone) as t(expertise,phone_num)
Hive Query
Select Name,Emp_id,expertise
from Employee
LATERAL VIEW explode(Subject) myTable1 as expertise
If there are two columns to be unnested then
Select Name,Emp_id,expertise,Phone
from Employee
LATERAL VIEW explode(Subject) myTable1 as expertise
LATERAL VIEW explode(Phone) myTable2 as Phone
Knowing when and how to do unnesting is very important as there will be multiple instances where a tough problem can be solved using Cross Join Unnest.
We have also created a video on Cross Join unnest to explain it in a better way, Do check out the
Video Link Here
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