MacLochlainns Weblog

I’ve been using Alan Beaulieu’s Learning SQL to teach my SQL Development class with MySQL 8. It’s a great book overall but Chapter 12 lacks a complete exercise. Here’s all that the author provides to the reader. This is inadequate for most readers to work with to solve the concept of a transaction.

Exercise 12-1

Generate a unit of work to transfer $50 from account 123 to account 789. You will need to insert two rows into the transaction table and update two rows in the account table. Use the following table definitions/data:

                      Account:
account_id     avail_balance    last_activity_date
-----------    --------------   ------------------
       123               450    2019-07-10 20:53:27
       789               125    2019-06-22 15:18:35
 
                      Transaction:
txn_id    txn_date      account_id    txn_type_cd    amount
------    ----------    -------+--    -----------    ------
  1001    2019-05-15           123    C                 500
  1002    2019-06-01           789    C                  75

Use txn_type_cd = ‘C” to indicate a credit (addition), and use txn_type_cd = ‘D’ to indicate a debit (substraction).

New Exercise 12-1

The problem with the exercise description is that the sakila database, which is used for most of the book, doesn’t have transaction or account tables. Nor, are there any instructions about general accounting practices or principles. These missing components make it hard for students to understand how to build the transaction.

The first thing the exercise’s problem defintion should qualify is how to create the account and transaction tables, like:

1. Create the account table, like this with an initial auto incrementing value of 1001:

   -- +--------------------+--------------+------+-----+---------+----------------+ -- | Field | Type | Null | Key | Default | Extra | -- +--------------------+--------------+------+-----+---------+----------------+ -- | account_id | int unsigned | NO | PRI | NULL | auto_increment | -- | avail_balance | double | NO | | NULL | | -- | last_activity_date | datetime | NO | | NULL | | -- +--------------------+--------------+------+-----+---------+----------------+

   -- +--------------------+--------------+------+-----+---------+----------------+ -- | Field | Type | Null | Key | Default | Extra | -- +--------------------+--------------+------+-----+---------+----------------+ -- | account_id | int unsigned | NO | PRI | NULL | auto_increment | -- | avail_balance | double | NO | | NULL | | -- | last_activity_date | datetime | NO | | NULL | | -- +--------------------+--------------+------+-----+---------+----------------+

2. Create the account table, like this with an initial auto incrementing value of 1001:

   -- +----------------+--------------+------+-----+---------+----------------+ -- | Field | Type | Null | Key | Default | Extra | -- +----------------+--------------+------+-----+---------+----------------+ -- | txn_id | int unsigned | NO | PRI | NULL | auto_increment | -- | txn_date | datetime | YES | | NULL | | -- | account_id | int unsigned | YES | | NULL | | -- | txn_type_cd | varchar(1) | NO | | NULL | | -- | amount | double | YES | | NULL | | -- +----------------+--------------+------+-----+---------+----------------+

   -- +----------------+--------------+------+-----+---------+----------------+ -- | Field | Type | Null | Key | Default | Extra | -- +----------------+--------------+------+-----+---------+----------------+ -- | txn_id | int unsigned | NO | PRI | NULL | auto_increment | -- | txn_date | datetime | YES | | NULL | | -- | account_id | int unsigned | YES | | NULL | | -- | txn_type_cd | varchar(1) | NO | | NULL | | -- | amount | double | YES | | NULL | | -- +----------------+--------------+------+-----+---------+----------------+

Checking accounts are liabilities to banks, which means you credit a liability account to increase its value and debit a liability to decrease its value. You should insert the initial rows into the account table with a zero avail_balance. Then, make these iniitial deposits:

1. Credit transaction table with an account_id column value of 123 with $500 and a txn_type_cd column value of ‘C’.
2. Credit transaction table with an account_id column value of 789 with $75 and a txn_type_cd column value of ‘C’.

Write an update statement to set the avail_balance column values equal to the aggregate sum of the transaction table’s rows, which treats credit transacctions (those with a ‘C’ in the txn_type_cd column as a positive number and thos with a ‘D’ in the txn_type_cd column as a negative number).

Generate a unit of work to transfer $50 from account 123 to account 789. You will need to insert two rows into the transaction table and update two rows in the account table. Use the following table definitions/data:

1. Debit transaction table with an account_id column value of 123 with $50 and a txn_type_cd column value of ‘D’.
2. Credit transaction table with an account_id column value of 789 with $50 and a txn_type_cd column value of ‘C’.

Apply the prior update statement to set the avail_balance column values equal to the aggregate sum of the transaction table’s rows, which treats credit transacctions (those with a ‘C’ in the txn_type_cd column as a positive number and thos with a ‘D’ in the txn_type_cd column as a negative number).

Here’s the solution to the problem:

-- +--------------------+--------------+------+-----+---------+----------------+
-- | Field              | Type         | Null | Key | Default | Extra          |
-- +--------------------+--------------+------+-----+---------+----------------+
-- | account_id         | int unsigned | NO   | PRI | NULL    | auto_increment |
-- | avail_balance      | double       | NO   |     | NULL    |                |
-- | last_activity_date | datetime     | NO   |     | NULL    |                |
-- +--------------------+--------------+------+-----+---------+----------------+
 
DROP TABLE IF EXISTS account, transaction;
 
CREATE TABLE account
( account_id          int unsigned PRIMARY KEY AUTO_INCREMENT
, avail_balance       double       NOT NULL
, last_activity_date  datetime     NOT NULL )
 ENGINE=InnoDB 
 AUTO_INCREMENT=1001 
 DEFAULT CHARSET=utf8mb4 
 COLLATE=utf8mb4_0900_ai_ci;
 
-- +----------------+--------------+------+-----+---------+----------------+
-- | Field          | Type         | Null | Key | Default | Extra          |
-- +----------------+--------------+------+-----+---------+----------------+
-- | txn_id         | int unsigned | NO   | PRI | NULL    | auto_increment |
-- | txn_date       | datetime     | YES  |     | NULL    |                |
-- | account_id     | int unsigned | YES  |     | NULL    |                |
-- | txn_type_cd    | varchar(1)   | NO   |     | NULL    |                |
-- | amount         | double       | YES  |     | NULL    |                |
-- +----------------+--------------+------+-----+---------+----------------+
 
CREATE TABLE transaction
( txn_id         int unsigned  PRIMARY KEY AUTO_INCREMENT
, txn_date       datetime      NOT NULL
, account_id     int unsigned  NOT NULL
, txn_type_cd    varchar(1)
, amount         double
, CONSTRAINT transaction_fk1 FOREIGN KEY (account_id)
 REFERENCES account(account_id))
 ENGINE=InnoDB
 AUTO_INCREMENT=1001
 DEFAULT CHARSET=utf8mb4
 COLLATE=utf8mb4_0900_ai_ci;
 
-- Insert initial accounts.
INSERT INTO account
( account_id
, avail_balance
, last_activity_date )
VALUES
( 123
, 0
,'2019-07-10 20:53:27');
 
INSERT INTO account
( account_id
, avail_balance
, last_activity_date )
VALUES
( 789
, 0
,'2019-06-22 15:18:35');
 
-- Insert initial deposits.
INSERT INTO transaction
( txn_date
, account_id
, txn_type_cd
, amount )
VALUES
( CAST(NOW() AS DATE)
, 123
,'C'
, 500 );
 
INSERT INTO transaction
( txn_date
, account_id
, txn_type_cd
, amount )
VALUES
( CAST(NOW() AS DATE)
, 789
,'C'
, 75 );
 
UPDATE account a
SET    a.avail_balance = 
 (SELECT  SUM(
            CASE
              WHEN t.txn_type_cd = 'C' THEN amount
              WHEN t.txn_type_cd = 'D' THEN amount * -1
            END) AS amount
 FROM     transaction t
 WHERE    t.account_id = a.account_id
 AND      t.account_id IN (123,789)
 GROUP BY t.account_id);
 
SELECT * FROM account;
SELECT * FROM transaction;
 
-- Insert initial deposits.
INSERT INTO transaction
( txn_date
, account_id
, txn_type_cd
, amount )
VALUES
( CAST(NOW() AS DATE)
, 123
,'D'
, 50 );
 
INSERT INTO transaction
( txn_date
, account_id
, txn_type_cd
, amount )
VALUES
( CAST(NOW() AS DATE)
, 789
,'C'
, 50 );
 
UPDATE account a
SET    a.avail_balance = 
 (SELECT  SUM(
            CASE
              WHEN t.txn_type_cd = 'C' THEN amount
              WHEN t.txn_type_cd = 'D' THEN amount * -1
            END) AS amount
 FROM     transaction t
 WHERE    t.account_id = a.account_id
 AND      t.account_id IN (123,789)
 GROUP BY t.account_id);
 
SELECT * FROM account;
SELECT * FROM transaction;

The results are:

+------------+---------------+---------------------+
| account_id | avail_balance | last_activity_date  |
+------------+---------------+---------------------+
|        123 |           450 | 2019-07-10 20:53:27 |
|        789 |           125 | 2019-06-22 15:18:35 |
+------------+---------------+---------------------+
2 rows in set (0.00 sec)
 
+--------+---------------------+------------+-------------+--------+
| txn_id | txn_date            | account_id | txn_type_cd | amount |
+--------+---------------------+------------+-------------+--------+
|   1001 | 2024-04-01 00:00:00 |        123 | C           |    500 |
|   1002 | 2024-04-01 00:00:00 |        789 | C           |     75 |
|   1003 | 2024-04-01 00:00:00 |        123 | D           |     50 |
|   1004 | 2024-04-01 00:00:00 |        789 | C           |     50 |
+--------+---------------------+------------+-------------+--------+
4 rows in set (0.00 sec)

As always, I hope this helps those trying to understand how CTEs can solve problems that would otherwise be coded in external imperative languages like Python.