MySQL：带条件的隐式连接：删除重复需要什么样的语句？
我正在处理JOIN语句（隐式），我已经将代码设置为无障碍地连接，并且当代码运行时，每个人都会得到相当多的副本。我想知道我应该用什么样的语句来只显示每个人中的一个 选择语句MySQL：带条件的隐式连接：删除重复需要什么样的语句？,mysql,sql,join,duplicates,implicit,Mysql,Sql,Join,Duplicates,Implicit,我正在处理JOIN语句（隐式），我已经将代码设置为无障碍地连接，并且当代码运行时，每个人都会得到相当多的副本。我想知道我应该用什么样的语句来只显示每个人中的一个 选择语句 SELECT CONCAT(customers.customer_first_name, ' ', customers.customer_last_name) as 'Customer', orders.order_date as 'Order Date', customers.customer_zip as
SELECT CONCAT(customers.customer_first_name, ' ', customers.customer_last_name) as 'Customer',
orders.order_date as 'Order Date', customers.customer_zip as 'Zipcode'
FROM customers, orders, order_details
WHERE order_details.item_id = 10
ORDER BY CONCAT(customers.customer_first_name, ' ', customers.customer_last_name) ASC;
/*Create Tables*/
CREATE TABLE customers
(
customer_id INT,
customer_first_name VARCHAR(20),
customer_last_name VARCHAR(20) NOT NULL,
customer_address VARCHAR(50) NOT NULL,
customer_city VARCHAR(20) NOT NULL,
customer_state CHAR(2) NOT NULL,
customer_zip CHAR(5) NOT NULL,
customer_phone CHAR(10) NOT NULL,
customer_fax CHAR(10),
CONSTRAINT customers_pk
PRIMARY KEY (customer_id)
);
CREATE TABLE artists
(
artist_id INT NOT NULL,
artist_name VARCHAR(30),
CONSTRAINT artist_pk
PRIMARY KEY (artist_id)
);
CREATE TABLE items
(
item_id INT NOT NULL,
title VARCHAR(50) NOT NULL,
artist_id INT NOT NULL,
unit_price DECIMAL(9,2) NOT NULL,
CONSTRAINT items_pk
PRIMARY KEY (item_id),
CONSTRAINT items_fk_artists
FOREIGN KEY (artist_id) REFERENCES artists (artist_id)
);
CREATE TABLE employees
(
employee_id INT NOT NULL,
last_name VARCHAR(20) NOT NULL,
first_name VARCHAR(20) NOT NULL,
manager_id INT,
CONSTRAINT employees_pk
PRIMARY KEY (employee_id),
CONSTRAINT emp_fk_mgr FOREIGN KEY (manager_id) REFERENCES employees(employee_id)
);
CREATE TABLE orders
(
order_id INT NOT NULL,
customer_id INT NOT NULL,
order_date DATE NOT NULL,
shipped_date DATE,
employee_id INT,
CONSTRAINT orders_pk
PRIMARY KEY (order_id),
CONSTRAINT orders_fk_customers
FOREIGN KEY (customer_id) REFERENCES customers (customer_id),
CONSTRAINT orders_fk_employees
FOREIGN KEY (employee_id) REFERENCES employees (employee_id)
);
CREATE TABLE order_details
(
order_id INT NOT NULL,
item_id INT NOT NULL,
order_qty INT NOT NULL,
CONSTRAINT order_details_pk
PRIMARY KEY (order_id, item_id),
CONSTRAINT order_details_fk_orders
FOREIGN KEY (order_id)
REFERENCES orders (order_id),
CONSTRAINT order_details_fk_items
FOREIGN KEY (item_id)
REFERENCES items (item_id)
);
/*Insert Statements*/
INSERT INTO customers VALUES
(1,'Korah','Blanca','1555 W Lane Ave','Columbus','OH','43221','6145554435','6145553928'),
(2,'Yash','Randall','11 E Rancho Madera Rd','Madison','WI','53707','2095551205','2095552262'),
(3,'Johnathon','Millerton','60 Madison Ave','New York','NY','10010','2125554800',NULL),
(4,'Mikayla','Davis','2021 K Street Nw','Washington','DC','20006','2025555561',NULL),
(5,'Kendall','Mayte','4775 E Miami River Rd','Cleves','OH','45002','5135553043',NULL),
(6,'Kaitlin','Hostlery','3250 Spring Grove Ave','Cincinnati','OH','45225','8005551957','8005552826'),
(7,'Derek','Chaddick','9022 E Merchant Wy','Fairfield','IA','52556','5155556130',NULL),
(8,'Deborah','Davis','415 E Olive Ave','Fresno','CA','93728','5595558060',NULL),
(9,'Karina','Lacy','882 W Easton Wy','Los Angeles','CA','90084','8005557000',NULL),
(10,'Kurt','Nickalus','28210N Avenue Stanford','Valencia','CA','91355','8055550584','055556689'),
(11,'Kelsey','Eulalia','7833 N Ridge Rd','Sacramento','CA','95887','2095557500','2095551302'),
(12,'Anders','Rohansen','12345 E 67th Ave NW','Takoma Park','MD','24512','3385556772',NULL),
(13,'Thalia','Neftaly','2508 W Shaw Ave','Fresno','CA','93711','5595556245',NULL),
(14,'Gonzalo','Keeton','12 Daniel Road','Fairfield','NJ','07004','2015559742',NULL),
(15,'Ania','Irvin','1099 N Farcourt St','Orange','CA','92807','7145559000',NULL),
(16,'Dakota','Baylee','1033 NSycamore Ave.','Los Angeles','CA','90038','2135554322',NULL),
(17,'Samuel','Jacobsen','3433 E Widget Ave','Palo Alto','CA','92711','4155553434',NULL),
(18,'Justin','Javen','828 S Broadway','Tarrytown','NY','10591','8005550037',NULL),
(19,'Kyle','Marissa','789 E Mercy Ave','Phoenix','AZ','85038','9475553900',NULL),
(20,'Erick','Kaleigh','Five Lakepointe Plaza, Ste 500','Charlotte','NC','28217','7045553500',NULL),
(21,'Marvin','Quintin','2677 Industrial Circle Dr','Columbus','OH','43260','6145558600','6145557580'),
(22,'Rashad','Holbrooke','3467 W Shaw Ave #103','Fresno','CA','93711','5595558625','5595558495'),
(23,'Trisha','Anum','627 Aviation Way','Manhatttan Beach','CA','90266','3105552732',NULL),
(24,'Julian','Carson','372 San Quentin','San Francisco','CA','94161','6175550700',NULL),
(25,'Kirsten','Story','2401 Wisconsin Ave NW','Washington','DC','20559','2065559115',NULL);
INSERT INTO artists(artist_id,artist_name) VALUES
(10,'Umani'),
(11,'The Ubernerds'),
(12,'No Rest For The Weary'),
(13,'Burt Ruggles'),
(14,'Sewed the Vest Pocket'),
(15,'Jess & Odie'),
(16,'Onn & Onn');
INSERT INTO items (item_id,title,artist_id,unit_price) VALUES
(1,'Umami In Concert',10,17.95),
(2,'Race Car Sounds',11,13),
(3,'No Rest For The Weary',12,16.95),
(4,'More Songs About Structures and Comestibles',12,17.95),
(5,'On The Road With Burt Ruggles',13,17.5),
(6,'No Fixed Address',14,16.95),
(7,'Rude Noises',15,13),
(8,'Burt Ruggles: An Intimate Portrait',13,17.95),
(9,'Zone Out With Umami',10,16.95),
(10,'Etcetera',16,17);
INSERT INTO employees VALUES
(1,'Smith','Cindy',null),
(2,'Jones','Elmer',1),
(3,'Simonian','Ralph',2),
(9,'Locario','Paulo',1),
(8,'Leary','Rhea',9),
(4,'Hernandez','Olivia',9),
(5,'Aaronsen','Robert',4),
(6,'Watson','Denise',8),
(7,'Hardy','Thomas',2);
INSERT INTO orders VALUES
(19,1,'2012-10-23','2012-10-28',6),
(29,8,'2012-11-05','2012-11-11',6),
(32,11,'2012-11-10','2012-11-13',NULL),
(45,2,'2012-11-25','2012-11-30',NULL),
(70,10,'2012-12-28','2013-01-07',5),
(89,22,'2013-01-20','2013-01-22',7),
(97,20,'2013-01-29','2013-02-02',5),
(118,3,'2013-02-24','2013-02-28',7),
(144,17,'2013-03-21','2013-03-29',NULL),
(158,9,'2013-04-04','2013-04-20',NULL),
(165,14,'2013-04-11','2013-04-13',NULL),
(180,24,'2013-04-25','2013-05-30',NULL),
(231,15,'2013-06-14','2013-06-22',NULL),
(242,23,'2013-06-24','2013-07-06',3),
(264,9,'2013-07-15','2013-07-18',6),
(298,18,'2013-08-18','2013-09-22',3),
(321,2,'2013-09-09','2013-10-05',6),
(381,7,'2013-11-08','2013-11-16',7),
(413,17,'2013-12-05','2014-01-11',7),
(442,5,'2013-12-28','2014-01-03',5),
(479,1,'2014-01-30','2014-03-03',3),
(491,16,'2014-02-08','2014-02-14',5),
(523,3,'2014-03-07','2014-03-15',3),
(548,2,'2014-03-22','2014-04-18',NULL),
(550,17,'2014-03-23','2014-04-03',NULL),
(601,16,'2014-04-21','2014-04-27',NULL),
(607,20,'2014-04-25','2014-05-04',NULL),
(624,2,'2014-05-04','2014-05-09',NULL),
(627,17,'2014-05-05','2014-05-10',NULL),
(630,20,'2014-05-08','2014-05-18',7),
(651,12,'2014-05-19','2014-06-02',7),
(658,12,'2014-05-23','2014-06-02',7),
(687,17,'2014-06-05','2014-06-08',NULL),
(693,9,'2014-06-07','2014-06-19',NULL),
(703,19,'2014-06-12','2014-06-19',7),
(778,13,'2014-07-12','2014-07-21',7),
(796,17,'2014-07-19','2014-07-26',5),
(800,19,'2014-07-21','2014-07-28',NULL),
(802,2,'2014-07-21','2014-07-31',NULL),
(824,1,'2014-08-01',NULL,NULL),
(827,18,'2014-08-02',NULL,NULL),
(829,9,'2014-08-02',NULL,NULL);
INSERT INTO order_details VALUES
(381,1,1),
(601,9,1),
(442,1,1),
(523,9,1),
(630,5,1),
(778,1,1),
(693,10,1),
(118,1,1),
(264,7,1),
(607,10,1),
(624,7,1),
(658,1,1),
(800,5,1),
(158,3,1),
(321,10,1),
(687,6,1),
(827,6,1),
(144,3,1),
(479,1,2),
(630,6,2),
(796,5,1),
(97,4,1),
(601,5,1),
(800,1,1),
(29,10,1),
(70,1,1),
(165,4,1),
(180,4,1),
(231,10,1),
(413,10,1),
(491,6,1),
(607,3,1),
(651,3,1),
(703,4,1),
(802,3,1),
(824,7,2),
(829,1,1),
(550,4,1),
(796,7,1),
(693,6,1),
(29,3,1),
(32,7,1),
(242,1,1),
(298,1,1),
(479,4,1),
(548,9,1),
(627,9,1),
(778,3,1),
(19,5,1),
(89,4,1),
(242,6,1),
(264,4,1),
(550,1,1),
(693,7,3),
(824,3,1),
(829,5,1),
(829,9,1);
看。。。现在你很可能得到一个笛卡尔积。抱歉，评论中没有此内容。。。新用户，分数还不足以发表评论。您拥有的是笛卡尔产品，而不是加入。您可以使用
distinct分组，但似乎您确实需要一个加入
。我正在为您编写类似的内容，但由于我不知道您的表
结构，我将猜测以下列：
SELECT CONCAT(customers.customer_first_name, ' ', customers.customer_last_name) as 'Customer',
orders.order_date as 'Order Date', customers.customer_zip as 'Zipcode'
FROM order_details
join orders
on order_details.order_id = orders.order_id and order_details.item_id = 10
join customers
on orders.customer_id = customers.customer_id
ORDER BY CONCAT(customers.customer_first_name, ' ', customers.customer_last_name) ASC;
当然，不能保证每个客户都有一个记录，因为我们，至少缺乏信息，不能假设没有客户有多个订单，每个订单都有一个订单详细信息，项目id=10如果您不想使用JOIN
关键字，您可以添加与WHERE
子句相关的关键列，如下所示：
SELECT CONCAT(customers.customer_first_name, ' ', customers.customer_last_name) as 'Customer',
orders.order_date as 'Order Date', customers.customer_zip as 'Zipcode'
FROM customers, orders, order_details
WHERE order_details.item_id = 10
AND orders.customer_id = customers.customer_id
AND order_details.order_id = orders.order_id
ORDER BY CONCAT(customers.customer_first_name, ' ', customers.customer_last_name) ASC;
是的，表格布局会很有帮助。通常，您必须将表连接到where
子句的关键列添加到其中，否则您将得到所有可能的行组合的交叉连接。我已经添加了这些列，在以后的问题中会记住它们。谢谢你的帮助。：）我一点也不介意。谢谢你的指导，我来看看。：）非常感谢您的帮助，但恐怕这只会给您更多的问题…所以，这不是一个隐含的加入。我的理解是，隐式连接的形式是不使用join关键字的。但如果不是，隐式连接是什么？这就是笛卡尔积（或笛卡尔积）的真正原因吗？哦，不过还是要谢谢你！这是非常有洞察力的，我已经弄明白了。它看起来确实比以前更合适。：）等等，我现在知道隐式的形式了。刚刚弄明白了。@Sierramagivney，你在做笛卡尔乘积，也叫隐式连接，但是，如果我们看一下在称为“web”的代数结构中定义的相遇/连接操作，那么连接是元素的一个常见乘法器，在本例中是关系，或者换句话说是表。然而，由于where子句中有过滤器，在所有记录找到公共乘法器（join）后（至少在理论上，但可能在实践中也是如此）执行过滤器，因此过滤器会减少它。这是有意义的，它回答了我的一个无声问题。感谢您的详细解释和更新您的答案。您的帮助很大，我们非常感谢。：）这是非常有用的！！你确实帮助我意识到我一直在做错事。非常感谢！！！：）虽然这个答案是正确的，但我们应该指出，这不如使用join关键字。在连接表时执行ON，与笛卡尔乘法相比节省了内存和时间。@LajosArpad。我们得比较一下执行计划。这是有人问过的，没有区别。不过我自己没有测试。