mysql 5.7 join

MySQL 5.7支持多种类型的join操作，包括：

1. 内连接（Inner Join）：只返回两个表中共有的行。
2. 外连接（Outer Join）：返回两个表中所有的行，同时匹配的行会被合并在一起。
3. 左连接（Left Join）：返回左表中所有的行以及右表中匹配的行。
4. 右连接（Right Join）：返回右表中所有的行以及左表中匹配的行。

下面是一个内连接的示例：

SELECT *
FROM table1
INNER JOIN table2
ON table1.id = table2.id;

这个查询将返回两个表中id相同的行。

注意，在MySQL 5.7中，可以使用JOIN关键字代替INNER JOIN关键字。例如，上面的查询可以写成：

SELECT *
FROM table1
JOIN table2
ON table1.id = table2.id;

相关问题

mysql5.7 递归查询

MySQL 5.7 不支持递归查询，但可以使用一些技巧来模拟递归查询。

一种常见的方法是使用连接查询（JOIN）和临时表。你可以创建一个临时表来存储中间结果，并在连接查询中使用该临时表。下面是一个示例：

CREATE TEMPORARY TABLE temp_table (id INT, parent_id INT);

INSERT INTO temp_table VALUES (1, NULL);
INSERT INTO temp_table VALUES (2, 1);
INSERT INTO temp_table VALUES (3, 2);
INSERT INTO temp_table VALUES (4, 3);

SELECT t1.id, t1.parent_id, t2.id as child_id
FROM temp_table t1
JOIN temp_table t2 ON t1.id = t2.parent_id;

这个查询会返回每个节点及其直接子节点的结果：

+----+-----------+----------+
| id | parent_id | child_id |
+----+-----------+----------+
|  1 |      NULL |        2 |
|  2 |         1 |        3 |
|  3 |         2 |        4 |
+----+-----------+----------+

如果你需要递归查询所有的子节点，可以使用多个连接查询来实现。但是，这种方法对于较深的递归查询可能会变得复杂和低效。

另一种方法是使用存储过程或递归公用表表达式（CTE）。然而，这些功能在 MySQL 5.7 中并不可用，而是从 MySQL 8.0 版本开始引入。

如果你需要执行复杂的递归查询，建议考虑升级到 MySQL 8.0 或使用其他支持递归查询的数据库管理系统。

mysql5.7 with recursive

MySQL 5.7 added support for recursive common table expressions (CTEs), which can be used to perform recursive queries. Recursive CTEs allow you to perform hierarchical queries on data, such as finding all the descendants of a particular node in a tree structure.

Here's an example of how to use recursive CTEs in MySQL 5.7:

Suppose we have a table called `employees` that has the following columns: `id`, `name`, and `manager_id`. The `manager_id` column contains the id of the employee's manager. We want to find all the employees who report to a given manager, including indirect reports.

We can use a recursive CTE to accomplish this as follows:

WITH RECURSIVE employee_tree AS (
    SELECT id, name, manager_id
    FROM employees
    WHERE id = <manager_id>
  UNION ALL
    SELECT e.id, e.name, e.manager_id
    FROM employees e
    JOIN employee_tree t ON e.manager_id = t.id
)
SELECT id, name
FROM employee_tree;

In this query, we define a recursive CTE called `employee_tree`. The initial query selects the employee with the given `manager_id`. The recursive part of the query joins the `employees` table with the `employee_tree` CTE on the `manager_id` column to find all the employees who report to the previously selected employees. This process continues recursively until no more matching rows are found.

Finally, we select the `id` and `name` columns from the `employee_tree` CTE to get the final result set.

Note that in order to use recursive CTEs, you must specify the `RECURSIVE` keyword after the `WITH` keyword.