MySQL 8.0

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CREATE TABLE tarif (
  tarif_id INT(11) NOT NULL AUTO_INCREMENT,
  start_tarif DATE NOT NULL,
  end_tarif DATE NOT NULL,
  day_tarif VARCHAR(50) NOT NULL,
  monday_tarif VARCHAR(50) NOT NULL,
  PRIMARY KEY (tarif_id)
);
INSERT INTO tarif VALUES 
  (1, '2019-02-01', '2019-02-10', '10', '5'),
  (2, '2019-02-11', '2019-02-20', '20', '5'),
  (3, '2019-02-21', '2019-02-28', '10', '5'),
  (4, '2019-03-01', '2019-02-10', '15', '5');

Records: 4  Duplicates: 0  Warnings: 0


CREATE TABLE calendar (
  dateValue DATE
);
INSERT INTO calendar
SELECT adddate('1970-01-01',t4*10000 + t3*1000 + t2*100 + t1*10 + t0) gen_date from
 (select 0 t0 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t0,
 (select 0 t1 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t1,
 (select 0 t2 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t2,
 (select 0 t3 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t3,
 (select 0 t4 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t4
having gen_date between '2019-01-01' and '2019-12-31'

Records: 365  Duplicates: 0  Warnings: 0


SELECT SUM(CASE WHEN DAYOFWEEK(cal.dateValue) = 2 THEN tarif.monday_tarif ELSE tarif.day_tarif END) AS sumWithMondayTarif 
FROM calendar cal
    LEFT JOIN tarif ON cal.dateValue BETWEEN start_tarif AND end_tarif
WHERE cal.dateValue BETWEEN '2019-02-05' AND '2019-02-21';


sumWithMondayTarif


240


SELECT SUM(CASE WHEN DAYOFWEEK(cal.dateValue) = 2 THEN tarif.monday_tarif ELSE tarif.day_tarif END) AS sumWithMondayTarif FROM (
    SELECT adddate('1970-01-01',t4*10000 + t3*1000 + t2*100 + t1*10 + t0) dateValue from
      (select 0 t0 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t0,
      (select 0 t1 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t1,
      (select 0 t2 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t2,
      (select 0 t3 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t3,
      (select 0 t4 union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) t4
      HAVING dateValue BETWEEN '2019-02-05' AND '2019-02-21'
) cal LEFT JOIN tarif ON cal.dateValue BETWEEN start_tarif AND end_tarif


sumWithMondayTarif


240