相信大家經常會遇到這樣的需求,通過一位職員的id/name獲取其下屬(包括非直屬)所有員工列表,用java實現這個功能相信也得花上一會功夫,但是如果是依賴數據庫來實現這個功能那就so easy了。先來看看Postgresql如何實現這樣的功能
WITH RECURSIVE r AS (SELECT * FROM t_account WHERE name = #{accountName}
union ALL
SELECT t_account.* FROM t_account, r WHERE t_account.parent = r.name
)
SELECT * FROM r ORDER BY name
這樣是不是在代碼量上減輕了很多啊,具體見postgresql官方文檔http://www.postgresql.org/docs/8.4/static/queries-with.html
接着來看看oracle如何做遞歸查詢:
從ROOT往末端遍歷:
select * from t_account t start with t.parent is null connect by prior t.name=t.parent
從末端到ROOT端遍歷:select * from t_account t start with t.name='**' connect by t.parent=t.name
具體用法細節請參考oracle文檔
下面再參照java的實現:
public class JsonTreeGenerate<T extends AbstractTreeNode> {
private Logger logger = Logger.getLogger(JsonTreeGenerate.class);
private Lock lock = new ReentrantLock();
private Set<T> set = new HashSet<T>();
public Set<T> getAllChild(Set<T> sets,T node){
lock.lock();
try {
if(set.size()>0){
set.clear();
}
recursionFn(sets,node);
} catch (Exception e) {
logger.error("", e);
}finally{
lock.unlock();
}
return set;
}
public void recursionFn(Set<T> sets , T node){
set.add(node);
if(hasChild(sets,node)){
List<T> hashSet = getChildList(sets , node);
Iterator<T> it = hashSet.iterator();
while(it.hasNext()){
T n = (T)it.next();
if(null==node.getChildren()){
node.setChildren(new ArrayList<AbstractTreeNode>());
}
node.getChildren().add(n);
recursionFn(sets,n);
}
//recursionFn(accountSet,node);
}
}
public List<T> getChildList(Set<T> list, T t){
List<T> nodeList=new ArrayList<T>();
Iterator<T> it = list.iterator();
while(it.hasNext()){
T accounts = it.next();
if(accounts.getParent()==t.getId()){
nodeList.add(accounts);
//t.getChildren().add(accounts);
}
}
return nodeList;
}
public boolean hasChild(Set<T> list,T node){
List<T> l =getChildList(list,node);
if(null!=l&&l.size()>0){
return true;
}
return false;
}
}
這個一比較就知道前者處理該問題的簡潔性了吧,