MySQL 8.0

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CREATE TABLE `cart` ( cart_id INT(11) NOT NULL AUTO_INCREMENT
, cart_id_product INT(11) NOT NULL
, cart_price DECIMAL(12,2) NOT NULL DEFAULT '0.00'
, cart_count INT(11) NOT NULL DEFAULT '1'
, cart_datetime DATETIME NOT NULL
, cart_ip VARCHAR(100) NOT NULL COLLATE 'utf8_general_ci'
, PRIMARY KEY (cart_id) USING BTREE ) COLLATE='utf8_general_ci' ENGINE=InnoDB AUTO_INCREMENT=6 ;
INSERt INTO cart VALUEs(20,1,12.1, 3,NOW(),'127.0.0.1')
UPDATE cart
SET cart_count = CASE
 WHEN cart_count >= 2 THEN cart_count  - 1
 ELSE 1
 END
WHERE cart_id = 20 AND cart_ip = '127.0.0.1';
SELECT * from cart


cart_id
cart_id_product
cart_price
cart_count
cart_datetime
cart_ip


20
1
12.10
2
2020-07-18 21:12:20
127.0.0.1


UPDATE cart
SET cart_count = CASE
 WHEN cart_count >= 2 THEN cart_count  - 1
 ELSE 1
 END
WHERE cart_id = 20 AND cart_ip = '127.0.0.1';
SELECT * from cart


cart_id
cart_id_product
cart_price
cart_count
cart_datetime
cart_ip


20
1
12.10
1
2020-07-18 21:12:20
127.0.0.1


UPDATE cart
SET cart_count = CASE
 WHEN cart_count >= 2 THEN cart_count  - 1
 ELSE 1
 END
WHERE cart_id = 20 AND cart_ip = '127.0.0.1';
SELECT * from cart


cart_id
cart_id_product
cart_price
cart_count
cart_datetime
cart_ip


20
1
12.10
1
2020-07-18 21:12:20
127.0.0.1