Merge pull request #1105 from yangj1211/cross-join

add doc of Cross join

docs/MatrixOne/Reference/SQL-Reference/Data-Query-Language/join/cross-join.md

@@ -0,0 +1,56 @@
+# **CROSS JOIN**
+
+## **语法说明**
+
+`CROSS JOIN` 用于实现两个表的笛卡尔积，也就是生成两个表中所有行的组合。
+
+## **语法结构**
+
+```
+>SELECT column_list
+FROM table1
+CROSS JOIN table2;
+```
+
+## **示例**
+
+```sql
+CREATE TABLE Colors (
+    color_id INT AUTO_INCREMENT,
+    color_name VARCHAR(50),
+    PRIMARY KEY (color_id)
+);
+
+CREATE TABLE Fruits (
+    fruit_id INT AUTO_INCREMENT,
+    fruit_name VARCHAR(50),
+    PRIMARY KEY (fruit_id)
+);
+
+INSERT INTO Colors (color_name) VALUES ('Red'), ('Green'), ('Blue');
+INSERT INTO Fruits (fruit_name) VALUES ('Apple'), ('Banana'), ('Cherry');
+
+mysql> SELECT c.color_name, f.fruit_name FROM Colors c CROSS JOIN Fruits f;--生成一个包含所有颜色和所有水果组合的结果集
++------------+------------+
+| color_name | fruit_name |
++------------+------------+
+| Red        | Apple      |
+| Green      | Apple      |
+| Blue       | Apple      |
+| Red        | Banana     |
+| Green      | Banana     |
+| Blue       | Banana     |
+| Red        | Cherry     |
+| Green      | Cherry     |
+| Blue       | Cherry     |
++------------+------------+
+9 rows in set (0.00 sec)
+
+mysql> SELECT c.color_name,f.fruit_name FROM Colors c CROSS JOIN Fruits f WHERE c.color_name = 'Red' AND f.fruit_name = 'Apple';--筛选出特定颜色和特定水果的组合
++------------+------------+
+| color_name | fruit_name |
++------------+------------+
+| Red        | Apple      |
++------------+------------+
+1 row in set (0.01 sec)
+```

mkdocs.yml

@@ -353,6 +353,7 @@ nav:
             - SUBQUERY with IN: MatrixOne/Reference/SQL-Reference/Data-Query-Language/subqueries/subquery-with-in.md
           - JOIN:
             - JOIN 概述: MatrixOne/Reference/SQL-Reference/Data-Query-Language/join/join.md
+            - CROSS JOIN: MatrixOne/Reference/SQL-Reference/Data-Query-Language/join/cross-join.md
             - INNER JOIN: MatrixOne/Reference/SQL-Reference/Data-Query-Language/join/inner-join.md
             - LEFT JOIN: MatrixOne/Reference/SQL-Reference/Data-Query-Language/join/left-join.md
             - RIGHT JOIN: MatrixOne/Reference/SQL-Reference/Data-Query-Language/join/right-join.md