1
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Would like feedback on my SQL (1) efficiency and (2) readability, and if the conditions I featured in my CASE statement could be transferred to a join, etc.

Here's the problem I'm solving:

Say you have a table bst with a column of nodes and column or corresponding parent nodes:

node   Parent
1       2
2       5
3       5
4       3
5       NULL

We want to write SQL such that we label each node as a “leaf”, “inner” or “Root” node, such that for the nodes above we get:

Node    Label
1       Leaf
2       Inner
3       Inner
4       Leaf
5       Root

Here's my solution:

WITH join_table AS 
(
SELECT 
    a.node a_node,
    a.parent a_parent,
    b.node b_node, 
    b.parent b_parent
 FROM
    bst a 
 LEFT JOIN 
    bst b ON a.parent = b.node 
 )

 SELECT 
    a_node as Node, 
    CASE 
        WHEN b_node IS NOT NULL and b_parent IS NOT NULL THEN 'Leaf'
        WHEN b_node IS NOT NULL and b_parent IS NULL THEN 'Inner'
        WHEN b_node IS NULL and b_parent IS NULL THEN 'Root'
        ELSE 'Error' 
    END AS Label 
 FROM 
    join_table 
\$\endgroup\$
  • 1
    \$\begingroup\$ What dialect of SQL are you targeting? (Apart from that, this is a really well-asked SQL question, so well done!) \$\endgroup\$ – Toby Speight Jan 30 at 16:58
  • \$\begingroup\$ @TobySpeight Let's just say Hive, but Postgres or MySQL would work too -- more focused on the underlying logic \$\endgroup\$ – zthomas.nc Jan 30 at 17:14
  • 1
    \$\begingroup\$ It is possible to rewrite your query without CASE, however, it will not help the performance since you are reading the whole tables anyway. The sequential scan is inevitable in this case. \$\endgroup\$ – Radim Bača Jan 30 at 19:17
  • \$\begingroup\$ @RadimBača Feel free to post such a response \$\endgroup\$ – zthomas.nc Jan 31 at 16:17
  • 1
    \$\begingroup\$ @zthomas.nc it would be a solution using UNION ALL which would lead to three sequential scans instead of one. \$\endgroup\$ – Radim Bača Jan 31 at 18:19

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