MySQL Inner Join 有限制吗？

我有这个查询来收集有关单个订单的信息，它变得非常复杂。

我没有任何数据可以测试，所以我问，如果有人在小型和大型数据集中有这方面的经验，那么您可以或应该在单个查询中进行多少个连接是否有限制？将大型查询分成较小的部分是否明智，或者这不会产生重大影响？

另外，拥有一个合法的WHERE每个之后的子句INNER JOIN?

谢谢你的建议。

这是查询：

# Order: Get Order

function getOrder($order_id) {
    $sql = "SELECT (order.id, order.created, o_status.status,
                    /* payment info */
                    order.total, p_status.status,
                    /* ordered by */
                    cust_title.title, cust.forename, cust.surname,
                    customer.phone, customer.email,
                    cust.door_name, cust.street1,
                    cust.street2, cust.town,
                    cust.city, cust.postcode,
                    /* deliver to */
                    recip_title.title, recipient.forename, recipient.surname,
                    recipient.door_name, recipient.street1,
                    recipient.street2, recipient.town,
                    recipient.city, recipient.postcode,
                    /* deliver info */
                        shipping.name, order.memo,
                    /* meta data */
                    order.last_update)
              FROM tbl_order AS order

        INNER JOIN tbl_order_st AS o_status
                ON order.order_status_id = o_status.id

        INNER JOIN tbl_payment_st AS p_status
                ON order.payment_status_id = p_status.id

        INNER JOIN (SELECT (cust_title.title, cust.forename, cust.surname,
                            customer.phone, customer.email,
        /* ordered by */    cust.door_name, cust.street1,
                            cust.street2, cust.town,
                            cust.city, cust.postcode)
                      FROM tbl_customer AS customer
                INNER JOIN tbl_contact AS cust
                          ON customer.contact_id = cust.id
                INNER JOIN tbl_contact_title AS cust_title
                        ON cust.contact_title_id = cust_title.id
                     WHERE order.customer_id = customer.id)
                ON order.customer_id = customer.id

        INNER JOIN (SELECT (recip_title.title, recipient.forename, recipient.surname,
        /* deliver to */    recipient.door_name, recipient.street1,
                            recipient.street2, recipient.town,
                            recipient.city, recipient.postcode)
                      FROM tbl_contact AS recipient
                INNER JOIN tbl_contact_title AS recip_title
                        ON recipient.contact_title_id = recip_title.id
                     WHERE order.contact_id = recipient.id)
                ON order.contact_id = recipient.id

        INNER JOIN tbl_shipping_opt AS shipping
                ON order.shipping_option_id = shipping.id

             WHERE order.id = '?';";
    dbQuery($sql, array((int)$order_id));
    $rows = dbRowsAffected();
    if ($rows == 1)
        return dbFetchAll();
    else
        return null;
}

由于有人请求此查询的架构，因此它是：

# TBL_CONTACT_TITLE

DROP TABLE IF EXISTS tbl_contact_title;
CREATE TABLE tbl_contact_title(
    id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY(id),
    title CHAR(3)
) ENGINE = InnoDB;
INSERT INTO tbl_contact_title
    (title)
VALUES  ('MR'),
    ('MRS'),
    ('MS');


# TBL_CONTACT

DROP TABLE IF EXISTS tbl_contact;
CREATE TABLE tbl_contact(
    id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY(id),
    contact_title_id INT,
    FOREIGN KEY(contact_title_id) REFERENCES tbl_contact_title(id) ON DELETE SET NULL,
    forename VARCHAR(50),
    surname VARCHAR(50),
    door_name VARCHAR(25),
    street1 VARCHAR(40),
    street2 VARCHAR(40),
    town VARCHAR(40),
    city VARCHAR(40),
    postcode VARCHAR(10),
    currency_id INT,
    FOREIGN KEY(currency_id) REFERENCES tbl_currency(id) ON DELETE SET NULL
) ENGINE = InnoDB;

# TBL_CUSTOMER

DROP TABLE IF EXISTS tbl_customer;
CREATE TABLE tbl_customer(
    id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY(id),
    contact_id INT,
    FOREIGN KEY(contact_id) REFERENCES tbl_contact(id) ON DELETE SET NULL,
    birthday DATE,
    is_male TINYINT,
    phone VARCHAR(20),
    email VARCHAR(50) NOT NULL
) ENGINE = InnoDB, AUTO_INCREMENT = 1000;

# TBL_ORDER_ST

DROP TABLE IF EXISTS tbl_order_st;
CREATE TABLE tbl_order_st(
    id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY(id),
    status VARCHAR(25)
) ENGINE = InnoDB;
INSERT INTO tbl_order_st
    (status)
VALUES
    ('NEW'),
    ('PROCESSING'),
    ('SHIPPED'),
    ('COMPLETED'),
    ('CANCELLED');


# TBL_SHIPPING_OPT

DROP TABLE IF EXISTS tbl_shipping_opt;
CREATE TABLE tbl_shipping_opt(
    id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY(id),
    name VARCHAR(50),
    description VARCHAR(255),
    cost DECIMAL(6,3)
) ENGINE = InnoDB;
INSERT INTO tbl_shipping_opt
    (name, description, cost)
VALUES
    ('UK Premier', 'U.K. Mainland upto 30KG, Next Working Day', 8.00),
    ('Europe Standard', 'Most European Destinations* upto 30KG, 2 to 5 Working Days *please check before purchase', 15.00);


# TBL_PAYMENT_ST

DROP TABLE IF EXISTS tbl_payment_st;
CREATE TABLE tbl_payment_st(
    id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY(id),
    status VARCHAR(25)
) ENGINE = InnoDB;
INSERT INTO tbl_payment_st
    (status)
VALUES
    ('UNPAID'),
    ('PAID');


# TBL_ORDER

DROP TABLE IF EXISTS tbl_order;
CREATE TABLE tbl_order(
    id INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY(id),
    customer_id INT,
        FOREIGN KEY(customer_id) REFERENCES tbl_customer(id) ON DELETE SET NULL,
    contact_id INT,
    FOREIGN KEY(contact_id) REFERENCES tbl_contact(id) ON DELETE SET NULL,
    created DATETIME,
    last_update TIMESTAMP,
    memo VARCHAR(255),
    order_status_id INT,
    FOREIGN KEY(order_status_id) REFERENCES tbl_order_st(id),
    shipping_option_id INT,
    FOREIGN KEY(shipping_option_id) REFERENCES tbl_shipping_opt(id),
    coupon_id INT,
    FOREIGN KEY(coupon_id) REFERENCES tbl_coupon(id) ON DELETE SET NULL,
    total DECIMAL(9,3),
    payment_status_id INT,
    FOREIGN KEY(payment_status_id) REFERENCES tbl_payment_st(id)
) ENGINE = InnoDB, AUTO_INCREMENT = 1000;

MySQL 的 JOIN 数几乎没有任何限制。你的加入人数还不错。但是，由于派生表没有索引，因此您所做的连接派生表（您的内部子查询）可能会导致性能问题。在没有索引的派生表上执行联接可能会很慢。

您应该考虑制作一个带有用于连接的索引的真正临时表，或者找出一种避免子查询的方法。

MySQL 中的 JOIN 基本上就像对每个连接行进行查找（查找）。因此，如果您要连接许多记录，MySQL 将必须执行许多查找。问题不在于您连接的表数量，而在于您连接的行数。

无论如何，MySQL 在放弃并读取整个表之前只会执行这么多次查找。它在决定哪个更便宜方面做得非常好。

也许您能做的最好的事情就是通过使用 ANALYZE TABLE 更新索引统计信息来帮助它猜测。

每个 SELECT 可以有一个 WHERE 子句。因此，您的内部子查询将有一个 WHERE 子句，您的外部查询将有一个 WHERE 子句，这些子查询将在 JOIN 之后应用（至少在逻辑上是这样，尽管 MySQL 通常会首先应用它们以提高性能）。

此外，所有这些都假设您知道如何正确使用索引。