MySQL 在插入前觸發檢查 email 是否存在

Question

I have designed a C# windows Form with textboxes that inserts a new employee or new customer with an inserted by textbox email, that checks by sql if that email exist to create a new user (if requires)

這 3 個表在 mysql 代碼中

CREATE TABLE users (
    idUser int AUTO_INCREMENT,
    emailUser varchar(50) not null unique,
    nameUser varchar(50) not null,
    userSince datetime,
    PRIMARY KEY (idUser)
);

CREATE TABLE customers (
    idCustomer int auto_increment,
    idUser int,
    typeCustomer int,
    PRIMARY KEY (idCustomer),
    FOREIGN KEY (idUser) REFERENCES users(idUser),
    FOREIGN KEY (typeCustomer) REFERENCES typeCustomer(idTypeCustomer)
);

CREATE TABLE employees (
    idEmployee int auto_increment,
    idUser int,
    typeEmployee int,
    PRIMARY KEY (idEmployee),
    FOREIGN KEY (idUser) REFERENCES users(idUser),
    FOREIGN KEY (typeEmployee) REFERENCES typeEmployee(idTypeEmployee)
);

所以我想知道 SQL 觸發器是否可以檢查用戶表中的所有行，以及如何檢查。 我還是不明白觸發器和程序>.<

Answer 1

觸發器將在記錄插入用戶后運行

CREATE TRIGGER after_users_insert
    AFTER INSERT
    ON users FOR EACH ROW
    BEGIN
        INSERT INTO customers (idUser , typeCustomer)
        VALUES(new.id, 1);
        INSERT INTO customers (employees , typeEmployee)
        VALUES(new.id, 1);
    END$$



Insert into users (emailUser, nameUser) 
SELECT 'test@email.com', 'test' WHERE NOT EXISTS (SELECT 1 FROM users  WHERE emailUser = 'test@email.com' )