How to Efficiently Join Tables with Comma-Separated Values?
Dec 24, 2024 am 05:45 AM
Joining Tables with Comma-Separated Column
When joining tables where one column contains comma-separated values, finding matching records can be challenging. To solve this issue, a viable approach is to utilize the find_in_set function.
Example Query with find_in_set
Suppose we have the following schema:
CREATE TABLE tblC (
id INT NOT NULL AUTO_INCREMENT,
nname VARCHAR(255),
userids VARCHAR(255),
PRIMARY KEY (id)
);
CREATE TABLE tblB (
id INT NOT NULL AUTO_INCREMENT,
username VARCHAR(255),
userid INT,
PRIMARY KEY (id)
);
INSERT INTO tblC (nname, userids) VALUES
('new1', '1,2'),
('new2', '1,3'),
('new3', '1,4'),
('new4', '3,2'),
('new5', '5,2');
INSERT INTO tblB (username, userid) VALUES
('A', 1),
('B', 2),
('C', 3),
('D', 4),
('E', 5);
To find all the usernames when searching for "new1," we can use the following query:
SELECT * FROM tblC AS c JOIN tblB AS b ON (find_in_set(b.userid, c.userids) > 0) WHERE c.nname = 'new1';
Normalization Consideration
However, it's important to note that storing comma-separated values in a single column is considered a violation of database normalization principles. A better approach would be to have a separate junction table to hold the user associations for each row in tblC. This would eliminate the need for find_in_set and allow for more efficient queries.
Normalized Schema Sample
CREATE TABLE tblC (
id INT NOT NULL AUTO_INCREMENT,
nname VARCHAR(255),
PRIMARY KEY (id)
);
CREATE TABLE tblC_user (
c_id INT,
userid INT,
PRIMARY KEY (c_id, userid)
);
INSERT INTO tblC (nname) VALUES
('new1'),
('new2'),
('new3'),
('new4'),
('new5');
INSERT INTO tblC_user (c_id, userid) VALUES
(1, 1),
(1, 2),
(2, 1),
(2, 3),
(3, 1),
(3, 4),
(4, 3),
(4, 2),
(5, 5),
(5, 2);
Query with Normalized Schema
SELECT * FROM tblC AS c JOIN tblC_user AS cu ON (c.id = cu.c_id) JOIN tblB AS b ON (b.userid = cu.userid) WHERE c.nname = 'new1';
By using a normalized schema, we optimize query performance and ensure data integrity by eliminating duplicate or inconsistent relationships.
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