with as 用法

查看( 15 ) / 评论( 0 ) / 评分( 0 / 0 )
with
sql1 as (select to_char(a) s_name from test_tempa),
sql2 as (select to_char(b) s_name from test_tempb where not exists (select s_name from sql1 where rownum=1))
select * from sql1
union all
select * from sql2
union all
select 'no records' from dual
where not exists (select s_name from sql1 where rownum=1)
and not exists (select s_name from sql2 where rownum=1);

再举个简单的例子

with a as (select * from test)

select * from a;

其实就是把一大堆重复用到的SQL语句放在with as 里面，取一个别名，后面的查询就可以用它

这样对于大批量的SQL语句起到一个优化的作用，而且清楚明了

这是搜索到的英文文档资料(说得比较全,但是本人英文特菜,还没具体了解到,希望各高手具体谈谈这个with
as 的好处)

About Oracle WITH clause
Starting in Oracle9i release 2 we see an incorporation of the SQL-99 “WITH clause”, a tool for materializing subqueries to save Oracle from having to re-compute them multiple times.

The SQL “WITH clause” is very similar to the use of Global temporary tables (GTT), a technique that is often used to improve query speed for complex subqueries. Here are some important notes about the Oracle “WITH clause”:

• The SQL “WITH clause” only works on Oracle 9i release 2 and beyond.
• Formally, the “WITH clause” is called subquery factoring
• The SQL “WITH clause” is used when a subquery is executed multiple times
• Also useful for recursive queries (SQL-99, but not Oracle SQL)

To keep it simple, the following example only references the aggregations once, where the SQL “WITH clause” is normally used when an aggregation is referenced multiple times in a query.
We can also use the SQL-99 “WITH clause” instead of temporary tables. The Oracle SQL “WITH clause” will compute the aggregation once, give it a name, and allow us to reference it (maybe multiple times), later in the query.

The SQL-99 “WITH clause” is very confusing at first because the SQL statement does not begin with the word SELECT. Instead, we use the “WITH clause” to start our SQL query, defining the aggregations, which can then be named in the main query as if they were “real” tables:

WITH
subquery_name
AS
(the aggregation SQL statement)
SELECT
(query naming subquery_name);

Retuning to our oversimplified example, let’s replace the temporary tables with the SQL “WITH clause”:

WITH
sum_sales AS
select /*+ materialize */
sum(quantity) all_sales from stores
number_stores AS
select /*+ materialize */
count(*) nbr_stores from stores
sales_by_store AS
select /*+ materialize */
store_name, sum(quantity) store_sales from
store natural join sales
SELECT
store_name
FROM
store,
sum_sales,
number_stores,
sales_by_store
where
store_sales > (all_sales / nbr_stores)
;

Note the use of the Oracle undocumented “materialize” hint in the “WITH clause”. The Oracle materialize hint is used to ensure that the Oracle cost-based optimizer materializes the temporary tables that are created inside the “WITH” clause. This is not necessary in Oracle10g, but it helps ensure that the tables are only created one time.

It should be noted that the “WITH clause” does not yet fully-functional within Oracle SQL and it does not yet support the use of “WITH clause” replacement for “CONNECT BY” when performing recursive queries.

To see how the “WITH clause” is used in ANSI SQL-99 syntax, here is an excerpt from Jonathan Gennick’s great work “Understanding the WITH Clause” showing the use of the SQL-99 “WITH clause” to traverse a recursive bill-of-materials hierarchy
The SQL-99 “WITH clause” is very confusing at first because the SQL statement does not begin with the word SELECT. Instead, we use the “WITH clause” to start our SQL query, defining the aggregations, which can then be named in the main query as if they were “real” tables:

WITH
subquery_name
AS
(the aggregation SQL statement)
SELECT
(query naming subquery_name);

Retuning to our oversimplified example, let’s replace the temporary tables with the SQL “WITH” clause”:

with as 用法相关推荐

c语言中external，static关键字用法
static用法: 在C中,static主要定义全局静态变量.定义局部静态变量.定义静态函数. 1.定义全局静态变量:在全局变量前面加上关键字static,该全局变量变成了全局静态变量.全局静态变量有 ...
Pandas_transform的用法
先来看一个实例问题. 如下销售数据中展现了三笔订单,每笔订单买了多种商品,求每种商品销售额占该笔订单总金额的比例.例如第一条数据的最终结果为:235.83 / (235.83+232.32+107.9 ...
Python中yield和yield from的用法
yield 后面接的是 future 对象调用方委托生成器 yield from 直接给出循环后的结果 yield from 委托者和子生成器直接通信 yield from 直接处理stopIte ...
pytorch学习中 torch.squeeze() 和torch.unsqueeze()的用法
squeeze的用法主要就是对数据的维度进行压缩或者解压. 先看torch.squeeze() 这个函数主要对数据的维度进行压缩,去掉维数为1的的维度,比如是一行或者一列这种,一个一行三列(1,3)的 ...
python yield 和 yield from用法总结
#例1. 简单输出斐波那契數列前 N 个数 #缺点:该函数可复用性较差,因为 fab 函数返回 None,其他函数无法获得该函数生成的数列 #要提高 fab 函数的可复用性,最好不要直接打印出数列,而 ...
tf.nn.embedding_lookup()的用法
函数: tf.nn.embedding_lookup( params, ids, partition_strategy='mod', name=None, validate_indices=True, ...
OpenMP用法大全
OpenMP基本概念 OpenMP是一种用于共享内存并行系统的多线程程序设计方案,支持的编程语言包括C.C++和Fortran.OpenMP提供了对并行算法的高层抽象描述,特别适合在多核CPU机器上的 ...
Dorado用法与示例
Dorado用法与示例 dorado用后总结一.dorado概念 dorado的产品全名是"dorado展现中间件".从产品形态上dorado由两部分组成,第一部分是一个具有AJ ...
TensorFlow用法
TensorFlow用法什么是TensorFlow TensorFlow是一个开源软件库,用于使用数据流图进行数值计算.图中的节点表示数学运算,而图的边缘表示流动的多维数据数组(张量).这种灵活的体 ...
TensorFlow Keras API用法
TensorFlow Keras API用法 Keras 是与 TensorFlow 一起使用的更高级别的作为后端的 API.添加层就像添加一行代码一样简单.在模型架构之后,使用一行代码,可以编译和拟 ...

with as 用法

with as 用法相关推荐

最新文章

热门文章