You can use a CASE statement to handle multiple if/then scenarios:

UPDATE table_to_update 
SET  cod_user= CASE WHEN user_rol = 'student' THEN '622057'
                   WHEN user_rol = 'assistant' THEN '2913659'
                   WHEN user_rol = 'admin' THEN '6160230'
               END
    ,date = '12082014'
WHERE user_rol IN ('student','assistant','admin')
  AND cod_office = '17389551';

BEGIN

DECLARE res INT;
DECLARE last_index INT;
SET @input := 'sales:  [  { id: 2, quantity: 5, price: 123, sum: 615 },  { id: 1, quantity: 9, price: 97, sum: 873 } ]';

SET @input:=REPLACE(@input, ' ', '');
SET @input:=REPLACE(@input, 'sales:', '');
SET @input:=REGEXP_REPLACE(@input, '([{,])([a-z])', '$1"$2');
SET @input:=REGEXP_REPLACE(@input, '([a-z]):', '$1":');

CREATE TEMPORARY TABLE new_tbl SELECT jtin.id AS jtin_id, jtin.quantity AS jtin_qty, pi.stock AS pi_qty, pi.price AS pi_price
FROM JSON_TABLE(@input,
                "$[*]"
                COLUMNS (id INT PATH "$.id" ,
                         quantity MEDIUMINT PATH "$.quantity",
                         price DECIMAL(7,2) PATH "$.price",
                         `sum` DECIMAL(8,2) PATH "$.sum"
                         )) AS jtin 
                LEFT JOIN prais_informations AS pi ON jtin.id  = pi.code;

SET res = (SELECT count(*) FROM new_tbl WHERE jtin_qty > pi_qty);

IF res = 0 THEN 
/*
  Инсертим новую запись, получившийся id, будет являтся id чека
*/
INSERT INTO checks(`data`, `user`) VALUES ( UNIX_TIMESTAMP(), 0);
SET last_index = (SELECT max(id) FROM checks);
/*
  Инсертим содержимое массива заказа в таблицу регистрации
*/
INSERT INTO sales(`code`, `quantity`, `price`, `sum`, `idCheck`) SELECT nt.jtin_id, nt.jtin_qty, nt.pi_price, (nt.jtin_qty * nt.pi_price), last_index FROM new_tbl AS nt;

/*
  Обновляем остатки на складе
*/

END IF

END

CREATE TABLE `sales` (
  `id` int UNSIGNED NOT NULL,
  `code` int UNSIGNED NOT NULL,
  `quantity` mediumint UNSIGNED NOT NULL,
  `price` decimal(7,2) NOT NULL,
  `sum` decimal(8,2) NOT NULL,
  `idCheck` int UNSIGNED NOT NULL,
  `data` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


--
ALTER TABLE `sales`
  ADD PRIMARY KEY (`id`),
  ADD KEY `idCheck` (`idCheck`);

--
ALTER TABLE `sales`
  MODIFY `id` int UNSIGNED NOT NULL AUTO_INCREMENT, AUTO_INCREMENT=0;

BEGIN

DECLARE res INT;
DECLARE last_index INT;
SET @input := 'sales:  [  { id: 2, quantity: 5, price: 123, sum: 615 },  { id: 1, quantity: 9, price: 97, sum: 873 } ]';

SET @input:=REPLACE(@input, ' ', '');
SET @input:=REPLACE(@input, 'sales:', '');
SET @input:=REGEXP_REPLACE(@input, '([{,])([a-z])', '$1"$2');
SET @input:=REGEXP_REPLACE(@input, '([a-z]):', '$1":');

CREATE TEMPORARY TABLE new_tbl SELECT jtin.id AS jtin_id, jtin.quantity AS jtin_qty, pi.stock AS pi_qty, pi.price AS pi_price
FROM JSON_TABLE(@input,
                "$[*]"
                COLUMNS (id INT PATH "$.id" ,
                         quantity MEDIUMINT PATH "$.quantity",
                         price DECIMAL(7,2) PATH "$.price",
                         `sum` DECIMAL(8,2) PATH "$.sum"
                         )) AS jtin 
                LEFT JOIN prais_informations AS pi ON jtin.id  = pi.code;

SET res = (SELECT count(*) FROM new_tbl WHERE jtin_qty > pi_qty);

IF res = 0 THEN 

INSERT INTO checks(`data`, `user`) VALUES ( UNIX_TIMESTAMP(), 0);

SET last_index = LAST_INSERT_ID();
/*
  Инсертим содержимое массива заказа в таблицу регистрации
*/
INSERT INTO sales(`code`, `quantity`, `price`, `sum`, `idCheck`) SELECT nt.jtin_id, nt.jtin_qty, nt.pi_price, (nt.jtin_qty * nt.pi_price), last_index FROM new_tbl AS nt;

UPDATE prais_informations AS pi LEFT JOIN new_tbl ON pi.code = new_tbl.jtin_id SET pi.stock = pi.stock - new_tbl.jtin_qty;

END IF;

END