id
1
2
3
4
5
Null
Null

Count(id)
Count (1)
Count('id')
Count(*)
Count(null)

В документации Postgresql:

count(*) -> bigint
number of input rows
count(expression) -> bigint
number of input rows for which the value of expression is not null

id  id
1   1
1   1
1   1
2   6
3   7
4   9

for id1 in table1:
  for id2 in table2:
    if id1 == id2:
      print(id1, id2)

select distinct name, row_number() over(order by id)
from table

select name, row_number() over(order by id)
from table

For Each value in pile1
    For Each value in pile2
        If pile1.value = pile2.value
        Return pile1.value, pile2.value

get first row R1 from   input 1
get first row R2 from   input 2

while not at the end   of either input
      begin
          if R1 joins with R2
              begin
                  get next row R2 from input 2
                  return (R1, R2)
              end
          else if R1 < R2
              get next row R1 from input 1
          else
              get next row R2 from input 2
    end

// Build phase
FOR each row in BuildTable DO
    Compute hash value for the join key
    Insert row into HashTable based on hash value
END FOR

// Probe phase
FOR each row in ProbeTable DO
    Compute hash value for the join key
    IF hash value exists in HashTable THEN
        Retrieve matching rows from HashTable
        FOR each matching row DO
            Combine rows from ProbeTable and BuildTable
            Add the combined row to the result set
        END FOR
    END IF
END FOR