英語 での Derived table の使用例とその 日本語 への翻訳
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Persistent Derived Table.
Above are some examples of joined tables and complex derived tables.
SQL-based Derived Table.
You cannot refer to LookML dimensions and measures in a SQL-based derived table.
Turning a SQL query into a derived table definition.
CREATE VIEW now has the capability to infer columnnames for views involving a GROUP BY clause or a derived table.
The Referencing Derived Tables in Other Derived Tables.
Using LookML to define a native derived table(NDT).
We can create a new derived table named customer_order_facts to do this:.
In cases where the query will take some time to run,a persistent derived table is often a better option.
Compared to SQL-based derived tables, they are much easier to read, understand, and reason about as you model your data.
The last two FROM clauses produce the same derived table from T1, T2, and T3.
Looker's derived table functionality provides a way to create newtables that don't already exist in your database.
You can then base a LookML view on the derived table, just like any other table. .
A persistent derived table(PDT) is written into a scratch schema on your database and regenerated on a schedule of your choosing.
The following list shows which dialects support derived tables in the most recent Looker release:.
The query expression can be embedded directly in the SQL statementtogether with a correlation name used to identify the derived table.
The Looker Block for a Derived Table Pattern allows user to create temporarytables to optimize table structure for….
If you have defined a datagroup for your caching policies,you can use the datagroup_trigger parameter to cause the persistent derived table(PDT) to rebuild on the same schedule as your caching policies.
When you add persistence to a derived table, it will exhibit some specific behaviors in Development Mode that you should know about.
This particular example couldbe done more efficiently in a single derived table, but it's useful for demonstrating derived table references.
SQL Derived Table& Common Table Expression Support SQL derived tables will enhance the ability of existing applications to use InterBase as a backend RDMBS.
For this reason, if you are defining a SQL-based derived table, you should make sure to alias each column with a clean name by using AS.
Because an ephemeral derived table runs a new query on your database every time a user requests data from it, you will want to be sure that the derived table is performant and won't put excessive strain on your database.
Although it's not always required, when referring to a derived table in this manner, it's often useful to alias the table. For example:.
To add persistence to our original derived table example, we could set it to rebuild when the order_datagroup datagroup triggers, and add indexes on both customer_id and first_order_date, like this:.
It is possible to reference one derived table in the definition of another, creating something called“cascading” derived tables. .
In this example we're creating a derived table from events that cleans out unwanted rows, and then creating a summary of that derived table.
There is no parameter that means“make this derived table persistent.” Rather, persistence is created by the addition of the datagroup_trigger, sql_trigger_value, or persist_for parameter.