image.png

1. 查詢

1.1. 得到節點 Node 下的所有節點，并按樹狀排序

SELECT * FROM tree WHERE lft BETWEEN Node.Lft AND Node.Rgt ORDER BY lft ASC;

1.2. 得到節點 Node 的路徑:

SELECT * FROM tree WHERE lft < Node.Lft AND rgt > Node.Rgt ORDER BY lft ASC;

1.3. 節點 Node 的子節點總數

(Node.Rgt-Node.Lft-1)/2

2. 修改

2.1. 增加一個節點

假設新節點的父節點是 Father，如果，這是它的第一個子節點，

nCurrent = Father.Lft;

否則，假設插入點的前一個兄弟為 Brother，

nCurrent = Brother.Rgt;

改變所有位于新節點右側的數值:

UPDATE tree SET rgt = rgt+2 WHERE rgt > nCurrent;
UPDATE tree SET lft = lft+2 WHERE lft > nCurrent;

這樣就為新插入的節點騰出了空間:

INSERT INTO tree(lft, rgt) VALUES(nCurrent+1, nCurrent+2);

2.2. 刪除一個節點 Node

DELETE Node;
UPDATE tree SET rgt = rgt - 2 WHERE rgt > Node.rgt;
UPDATE tree SET lft = lft - 2 WHERE lft > Node.rgt;

2.3. 子樹 SubTree 從 Father1 遷移至 Father2

相當于先插入子樹，再刪掉原來的子樹。
子樹根的左右值:

nSubTreeLeft = SubTree.Lft;
nSubTreeRight = SubTree.Rgt;

子樹上所有的節點數(包括子樹的根):

nSubTreeNodeNum = (nSubTreeRight - nSubTreeLeft - 1) / 2 + 1;

如果，這是 Father2 的第一個子節點，

nCurrent = Father2.Lft; 

否則，假設插入點的前一個兄弟為 Brother，

nCurrent = Brother.Rgt;

騰子樹空間:

UPDATE tree SET lft=lft + 2* nSubTreeNodeNum WHERE lft > nCurrent; UPDATE tree SET rgt=rgt + 2* nSubTreeNodeNum WHERE rgt > nCurrent;

移動子樹:

UPDATE tree SET lft = lft + nCurrent + 1 - nSubTreeLeft, rgt = rgt + nCurrent + 1 - nSubTreeLeft WHERE lft BETWEEN nSubTreeLeft AND nSubTreeRight;

刪掉原來子樹占的空間:

UPDATE tree SET lft=lft -2* nSubTreeNodeNum WHERE lft > nSubTreeRight; 
UPDATE tree SET rgt=rgt -2* nSubTreeNodeNum WHERE rgt > nSubTreeRight;

重構左右值

假設表結構類似:

image.png

/**
     * 重構左右值
     *
     * @param rootId 根節點id
     * @param left   左值開始值
     * @return
     */
    public int rebuildTree(int rootId, int left) {
        int right = left + 1;
        List<Category> categories = categoryMapper.selectByParentCategoryID(rootId);
        for (Category category : categories) {
            right = rebuildTree(category.getId(), right);
        }
        Category category = categoryMapper.selectByPrimaryKey(rootId);
        category.setLft(left);
        category.setRgt(right);
        categoryMapper.updateByPrimaryKey(category);
        return right + 1;
    }