how to perform sql actions/query for duplicate rows - sql

Question

I have 2 tables:
1-brokers(this is a company that could have multiple broker individuals)
and
2-brokerIndividuals (A person/individuals table that has a foreign key of broker company it belongs to and the individuals details)
I'm trying to create a unique index column for brokers table where the fields companyName are unique and isDeleted is NULL. Currently, the table is already populated so I want to write an SQL QUERY to find duplicate rows and whenever there are rows with the same companyName and isDeleted=NULL, I would like to perform 2 actions/queries:
1-keep the first row as it is and changes other duplicates(rows following the first duplicate) rows' isDeleted columns value to true.
2- associate or change the foreign key in brokerIndividuals for the duplicate rows for the first row.
The verbal description of what I am trying to do is: soft delete the duplicate rows and associate their corresponding brokerIndividuals to the first occurrence of duplicates. Table needs to have 1 occurrence of companyName where isDeleted is NULL.
I am using knex.js ORM so if that help's you can also suggest a solution using knex functions but knex doesn't support partial index yet( Knex.js - How to create unique index with 'where' clause? ) so I have to use the raw SQL method. Plus the DB I'm using is mssql(version: 6.0.1).

Answer 1

Here's a full test case (commented), with link to the fiddle:
Working test case, tested with MySQL 5.5, 5.6, 5.7, 8.0 and MariaDB up to 10.6
Create the tables and insert initial data with duplicate company_name entries:
CREATE TABLE brokers (
id int primary key auto_increment
, company_name VARCHAR(30)
, isDeleted boolean default null
);
CREATE TABLE brokerIndividuals (
id int primary key auto_increment
, broker_id int references brokers (id)
);
INSERT INTO brokers (company_name) VALUES
('name1')
, ('name1')
, ('name1')
, ('name1')
, ('name123')
, ('name123')
, ('name123')
, ('name123')
;
INSERT INTO brokerIndividuals (broker_id) VALUES
(2)
, (7)
;
SELECT * FROM brokers;
+----+--------------+-----------+
| id | company_name | isDeleted |
+----+--------------+-----------+
| 1 | name1 | null |
| 2 | name1 | null |
| 3 | name1 | null |
| 4 | name1 | null |
| 5 | name123 | null |
| 6 | name123 | null |
| 7 | name123 | null |
| 8 | name123 | null |
+----+--------------+-----------+
SELECT * FROM brokerIndividuals;
+----+-----------+
| id | broker_id |
+----+-----------+
| 1 | 2 |
| 2 | 7 |
+----+-----------+
Adjust brokers to determine isDeleted based on the MIN(id) per company_name:
UPDATE brokers
JOIN (
SELECT company_name, MIN(id) AS id
FROM brokers
GROUP BY company_name
) AS x
ON x.company_name = brokers.company_name
AND isDeleted IS NULL
SET isDeleted = CASE WHEN (x.id <> brokers.id) THEN 1 END
;
The updated brokers contents:
SELECT * FROM brokers;
+----+--------------+-----------+
| id | company_name | isDeleted |
+----+--------------+-----------+
| 1 | name1 | null |
| 2 | name1 | 1 |
| 3 | name1 | 1 |
| 4 | name1 | 1 |
| 5 | name123 | null |
| 6 | name123 | 1 |
| 7 | name123 | 1 |
| 8 | name123 | 1 |
+----+--------------+-----------+
For brokerIndividuals, find / set the correct broker_id:
UPDATE brokerIndividuals
JOIN brokers AS b1
ON b1.id = brokerIndividuals.broker_id
JOIN brokers AS b2
ON b1.company_name = b2.company_name
AND b2.isDeleted IS NULL
SET brokerIndividuals.broker_id = b2.id
;
New contents:
SELECT * FROM brokerIndividuals;
+----+-----------+
| id | broker_id |
+----+-----------+
| 1 | 1 |
| 2 | 5 |
+----+-----------+