swanandp · November 29, 2023 17:23 · Nov 29, 2023 · Nov 29, 2023 · Nov 29, 2023 · Nov 29, 2023
diff --git a/recurstive_cte.sql → 1_recurstive_cte.sql b/recurstive_cte.sql → 1_recurstive_cte.sql
diff --git a/how_does_it_work.md → 2_how_does_it_work.md b/how_does_it_work.md → 2_how_does_it_work.md
diff --git a/how_does_it_work.md b/how_does_it_work.md
@@ -0,0 +1,15 @@
+# How does it work?
+
+The general format of a recursive query is:
+
+```sql
+WITH RECURSIVE cte_name(arg1, arg2, arg3) AS( -- arg1, arg2 are column outputs from seed_data_query
+    seed_data_query -- non-recursive term, executes first
+    UNION ALL -- can be just UNION, which returns unique rows. It's slower.
+    recursive_query_based_on_seed_data  -- recursive term
+) SELECT * FROM cte_name;
+```
+When executing, the `seed_data_query` executes first, giving a set of base results. Note that this is not the "base case" from recursion. In recursion, base case is a terminating clause and is the last to execute. Here, the non-recursive part acts as seed data for the recursive part. Termination of a recursive CTE happens when the recursive part yields 0 rows.
+
+The result of `seed_data_query` is passed as argument to the next query. For `recursive_query_based_on_seed_data`, `cte_name(arg1, arg2, arg3)` is available as a temporary table with columns arg1, arg2, arg3, etc. All your regular join operations are possible with this temporary table.
+
diff --git a/recurstive_cte.sql b/recurstive_cte.sql
@@ -5,7 +5,7 @@ CREATE TABLE posts
     reply_to_id bigint REFERENCES posts (id)
 );
 
-TRUNCATE posts RESTART IDENTITY; -- this deleted all rows and restarts the auto-increment ID
+TRUNCATE posts RESTART IDENTITY; -- this deletes all rows and restarts the auto-increment ID
 
 /*
 Let's create nested data like this:

diff --git a/recurstive_cte.sql b/recurstive_cte.sql
@@ -0,0 +1,91 @@
+CREATE TABLE posts
+(
+    id          bigserial PRIMARY KEY,
+    content     text NOT NULL,
+    reply_to_id bigint REFERENCES posts (id)
+);
+
+TRUNCATE posts RESTART IDENTITY; -- this deleted all rows and restarts the auto-increment ID
+
+/*
+Let's create nested data like this:
+-> R
+    -> R-O
+        -> R-O-A
+            -> R-O-A-D
+        -> R-O-D
+    -> R-A
+        -> R-A-P
+-> S
+    -> H
+*/
+
+INSERT INTO posts(content)
+VALUES ('R');
+
+INSERT INTO posts(content, reply_to_id)
+VALUES ('RO', 1),
+       ('RA', 1);
+
+INSERT INTO posts(content, reply_to_id)
+VALUES ('ROA', 2),
+       ('RAP', 3);
+
+INSERT INTO posts(content, reply_to_id)
+VALUES ('ROAD', 4);
+
+INSERT INTO posts(content, reply_to_id)
+VALUES ('ROD', 2);
+
+INSERT INTO posts(content)
+VALUES ('S');
+
+INSERT INTO posts(content, reply_to_id)
+VALUES ('SH', 7);
+
+SELECT *
+FROM posts;
+
+
+-- select all children for 'R'
+WITH RECURSIVE leaves(id, content, reply_to_id) AS
+                   (SELECT id, content, reply_to_id
+                    FROM posts
+                    WHERE content = 'R'
+
+                    UNION ALL
+
+                    SELECT p.id, p.content, p.reply_to_id
+                    FROM leaves l
+                             INNER JOIN posts p ON l.id = p.reply_to_id)
+SELECT *
+FROM leaves;
+
+-- select all children for 'S'
+WITH RECURSIVE leaves(id, content, reply_to_id) AS
+                   (SELECT id, content, reply_to_id
+                    FROM posts
+                    WHERE content = 'S'
+
+                    UNION ALL
+
+                    SELECT p.id, p.content, p.reply_to_id
+                    FROM leaves l
+                             INNER JOIN posts p ON l.id = p.reply_to_id)
+SELECT *
+FROM leaves;
+
+-- select all children for 'RO'
+WITH RECURSIVE leaves(id, content, reply_to_id) AS
+                   (SELECT id, content, reply_to_id
+                    FROM posts
+                    WHERE content = 'RO'
+
+                    UNION ALL
+
+                    SELECT p.id, p.content, p.reply_to_id
+                    FROM leaves l
+                             INNER JOIN posts p ON l.id = p.reply_to_id)
+SELECT *
+FROM leaves;
+