SQL Serise Part IV (SQL Subquery)

Subquery

SELECT DATE_TRUNC('day',createTime) AS day, col, COUNT(*) as new_col
FROM table
GROUP BY 1,2
ORDER BY 3 DESC;
# same result as above statement

SELECT *
FROM (SELECT DATE_TRUNC('day',createTime AS day,
             col, COUNT(*) new_col
      FROM table
      GROUP BY 1,2
      ORDER BY 3 DESC) sub;

# more complicated statement
SELECT SUM(total_col)
FROM table
WHERE DATE_TRUNC('month', createTime) = 
      (SELECT DATE_TRUNC('month', MIN(createTime)) FROM table);

SELECT AVG(avg_amount)
FROM (SELECT o.account_id, AVG(o.total_amount) avg_amount
      FROM orders o
      GROUP BY 1
      HAVING AVG(o.total_amount) > (SELECT AVG(o.total_amount) avg_all
                                     FROM orders o
                                     JOIN accounts a
                                     ON a.id = o.account_id)) temp_table;

WITH

WITH new_table AS (
          SELECT DATE_TRUNC('day',createTime) AS day, 
                        col, COUNT(*) as new_col
          FROM table 
          GROUP BY 1,2)

SELECT channel, AVG(new_col) AS average_col
FROM new_table
GROUP BY 1
ORDER BY 2 DESC;