How to Split Hours between Min(Date) and Max(Date) into 4 Hours Slab in MySQL Server

My Table :

CREATE TABLE `tbdata` (
  `ID` INT(10) NOT NULL AUTO_INCREMENT,
  `PatientID` INT(10) NOT NULL,
  `RecordDate` DATETIME NOT NULL,
  PRIMARY KEY (`ID`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=latin1;

Data in Tables looks like below :

ID  PatientID RecordDate
132    172116 2019—11—15 13:43:45
133    172816 2019—11—15 13:43:55
134    172816 2019—11—15 13:44:06
135    172816 2019—11—15 13:44:16
136    172816 2019—11—15 13:44:27
137    172816 2019—11—15 13:44:38
138    172816 2019—11—15 13:44:48
139    172816 2019—11—15 13:44:59
140    172816 2019—11—15 13:45:09
141    172816 2019—11—15 13:45:20
142    172816 2019—11—15 13:45:31
143    172816 2019—11—15 13:45:41
144    172816 2019—11—15 13:45:52
145    172816 2019—11—15 13:46:02
146    172816 2019—11—15 13:46:13
147    172816 2019—11—15 13:46:24
148    172816 2019—11—15 13:46:35
149    172816 2019—11—15 13:46:46
150    172816 2019—11—15 13:46:56
151    172816 2019—11—15 13:47:07
152    172816 2019—11—15 13:47:19
153    172816 2019—11—15 13:47:30
154    172816 2019—11—15 13:47:43
155    172816 2019—11—15 13:47:54
156    172816 2019—11—15 13:48:04
157    172816 2019—11—15 13:48:15
158    172816 2019—11—15 14:48:25
159    172816 2019—11—15 17:48:36
160    172816 2019—11—15 18:48:47
161    172816 2019—11—15 20:48:57
162    172816 2019—11—15 22:49:08
163    172816 2019—11—15 23:49:18

Required Result like :

4 Hours slot :
start from min(RecordDate)
2019-11-15 13:00:00 - 2019-11-15 17:00:00
2019-11-15 17:00:00 - 2019-11-15 21:00:00
2019-11-15 21:00:00 - 2019-11-16 01:00:00
... till max(RecordDate)

Answer

I do not know how well this performs but it does return what you want:

EDIT I have updated it to filter out intervals with no data in the table.

**Schema (MySQL v8.0)**

    CREATE TABLE `tbdata` (
      `ID` INT(10) NOT NULL AUTO_INCREMENT,
      `PatientID` INT(10) NOT NULL,
      `RecordDate` DATETIME NOT NULL,
      PRIMARY KEY (`ID`)
    ) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=latin1;

    INSERT INTO tbdata (PatientID, RecordDate)
    VALUES
    (172116, '2019-11-15 13:43:45'),
    (172816, '2019-11-15 13:43:55'),
    (172816, '2019-11-15 13:44:06'),
    (172816, '2019-11-15 13:44:16'),
    (172816, '2019-11-15 13:44:27'),
    (172816, '2019-11-15 13:44:38'),
    (172816, '2019-11-15 13:44:48'),
    (172816, '2019-11-15 13:44:59'),
    (172816, '2019-11-15 13:45:09'),
    (172816, '2019-11-15 13:45:20'),
    (172816, '2019-11-15 13:45:31'),
    (172816, '2019-11-15 13:45:41'),
    (172816, '2019-11-15 13:45:52'),
    (172816, '2019-11-15 13:46:02'),
    (172816, '2019-11-15 13:46:13'),
    (172816, '2019-11-15 13:46:24'),
    (172816, '2019-11-15 13:46:35'),
    (172816, '2019-11-15 13:46:46'),
    (172816, '2019-11-15 13:46:56'),
    (172816, '2019-11-15 13:47:07'),
    (172816, '2019-11-15 13:47:19'),
    (172816, '2019-11-15 13:47:30'),
    (172816, '2019-11-15 13:47:43'),
    (172816, '2019-11-15 13:47:54'),
    (172816, '2019-11-15 13:48:04'),
    (172816, '2019-11-15 13:48:15'),
    (172816, '2019-11-15 14:48:25'),
    (172816, '2019-11-15 17:48:36'),
    (172816, '2019-11-15 18:48:47'),
    (172816, '2019-11-15 20:48:57'),
    (172816, '2019-11-15 22:49:08'),
    (172816, '2019-11-15 23:49:18');

---

    DELIMITER //
    CREATE PROCEDURE While_Loop()
    BEGIN
        SET @row_number = 0;

        SELECT MIN(RecordDate), MAX(RecordDate)
        INTO @min, @max
        FROM tbdata;

        SET @min = DATE_FORMAT(@min, "%Y-%m-%d %H:00:00");

        SELECT CEIL(TIMESTAMPDIFF(HOUR, @min, @max)/4) INTO @number_of_intervals;

        WHILE @row_number <= @number_of_intervals DO
        SET @start_time = @min;
        SET @end_time = DATE_ADD(@start_time, INTERVAL 4 HOUR);
        SET @min = @end_time;
        SET @row_number = @row_number + 1;

        SELECT COUNT(PatientID) INTO @existent_slot FROM tbdata WHERE RecordDate BETWEEN @start_time and @end_time;

          IF @existent_slot THEN
          SELECT @start_time start_time, @end_time end_time;
          END IF;
        END WHILE;


    END;

---

**Query #1**

    CALL While_Loop();

| start_time          | end_time            |
| ------------------- | ------------------- |
| 2019-11-15 13:00:00 | 2019-11-15 17:00:00 |

| start_time          | end_time            |
| ------------------- | ------------------- |
| 2019-11-15 17:00:00 | 2019-11-15 21:00:00 |

| start_time          | end_time            |
| ------------------- | ------------------- |
| 2019-11-15 21:00:00 | 2019-11-16 01:00:00 |

---

You should probably store the results in a temporary table instead of just printing them out.

Below updated Code works for me.

DELIMITER $$

DROP PROCEDURE IF EXISTS `While_Loop`$$

CREATE PROCEDURE `While_Loop`()
BEGIN
    DROP TEMPORARY TABLE IF EXISTS _tempTableTimeSlot;
    CREATE TEMPORARY TABLE _tempTableTimeSlot(
        starttime VARCHAR(100),
        endtime VARCHAR(100)
    );
        SET @row_number = 0;
        SELECT MIN(RecordDate), MAX(RecordDate)
        INTO @min, @max
        FROM tbdata;
        SET @min = DATE_FORMAT(@min, "%Y-%m-%d %H:00:00");
        SELECT CEIL(TIMESTAMPDIFF(HOUR, @min, @max)/4) INTO @number_of_intervals;
        WHILE @row_number <= @number_of_intervals DO
        SET @start_time = @min;
        SET @end_time = DATE_ADD(@start_time, INTERVAL 4 HOUR);
        SET @min = @end_time;
        SET @row_number = @row_number + 1;

    SELECT COUNT(PatientID) INTO @existent_slot FROM tbdata WHERE RecordDate BETWEEN @start_time AND @end_time;
        IF @existent_slot THEN
            INSERT INTO _tempTableTimeSlot(starttime,endtime) SELECT @start_time start_time1, @end_time end_time1;
        END IF; 
        END WHILE;

      SELECT * FROM _tempTable;
      DROP TEMPORARY TABLE IF EXISTS _tempTableTimeSlot;  
END$$

DELIMITER ;