I have a SQL query that loops data - mysql

Question

I wrote a query to display certain record, but it is displaying extra data, for instance I have only 239 records in my database, but the query is displaying 356 records. Can anyone advice me on what I did wrong, I would really appreciate it. Here is the query:
SELECT DISTINCT
t.branchid,
t.occupancyid,
t.wardnumber,
t.bednumber,
t.admissiondate,
ti.patientname
FROM
bedoccupancydetail t
JOIN
consultationheader ti ON t.occupancyid = ti.occupancyid
WHERE
t.checkedout = '0'

Answer 1

There might not be any problem with your query, just it is how mysql (or any RDBMS) behaves. In your case in the two tables bedoccupancydetail and consultationheader are joined by occupancyid and it seems this columns is not unique and contains duplicate values, for each matching (duplicate) record it adds a row/record after joining.
Let's see the below example which I run at https://www.tutorialspoint.com/execute_sql_online.php:
BEGIN TRANSACTION;
CREATE TABLE NAMES(Id integer PRIMARY KEY, Name text);
INSERT INTO NAMES VALUES(1,'Tom');
INSERT INTO NAMES VALUES(2,'Lucy');
INSERT INTO NAMES VALUES(3,'TOM');
INSERT INTO NAMES VALUES(4,'TOM');
CREATE TABLE ABC(Id integer PRIMARY KEY, Name text, Another text);
INSERT INTO ABC VALUES(1,'Tom', 'A');
INSERT INTO ABC VALUES(2,'Lucy', 'B');
INSERT INTO ABC VALUES(3,'TOM', 'C');
INSERT INTO ABC VALUES(4,'TOM', 'D');
COMMIT;
/* Display all the records from the table */
SELECT ABC.Name, NAMES.Name, ABC.Another
FROM NAMES
JOIN ABC on ABC.Name = NAMES.Name;
As you see each table has 4 rows but the result has 6 rows:
$sqlite3 database.sdb < main.sql
Tom|Tom|A
Lucy|Lucy|B
TOM|TOM|C
TOM|TOM|D
TOM|TOM|C
TOM|TOM|D