SQL best practices: a brief guide to writing better SQL queries.This article covers some best practices for writing SQL queries for data analysts and data scientists. Most of our discussion will concern SQL in general, but we’ll include some notes on features specific to Metabase that make ...
Eine SQL-Subquery ist eine Abfrage, die in eine andere SQL-Abfrage eingebettet ist. Sie wird verwendet, um Operationen durchzuführen, die mehrere Schritte oder eine komplexe Logik erfordern. Zu den Aufgaben von Unterabfragen in SQL gehören die folgenden: Filtern von Datensätzen anhand...
在Calcite Optimizer 里,结合了经典的优化思路:首先是对 LogicalPlan 进行一些确定性的改写,例如 SubQuery 改写、解关联、常量折叠等。然后做一些通用的改写,例如各种 push down,形成 FlinkLogicalPlan。这部分优化是流批通用的。然后流和批根据各自底层的实现,进行特定的优化,例如批上会根据 Cost 选择不同的 Join 算...
在Calcite Optimizer 里,结合了经典的优化思路:首先是对 LogicalPlan 进行一些确定性的改写,例如 SubQuery 改写、解关联、常量折叠等。然后做一些通用的改写,例如各种 push down,形成 FlinkLogicalPlan。这部分优化是流批通用的。然后流和批根据各自底层的实现,进行特定的优化,例如批上会根据 Cost 选择不同的 Join 算...
Do not use a subquery or its JOIN condition in an OR expression. Do not use scalar subqueries containing LIMIT, for example, SELECT (SELECT x FROM t2 WHERE t2.id= t.id LIMIT 1),a,b FROM t. If subqueries and primary table are routed to the same shard, add /*+db=xxx*/ before ...
在Calcite Optimizer 里,结合了经典的优化思路:首先是对 LogicalPlan 进行一些确定性的改写,例如 SubQuery 改写、解关联、常量折叠等。然后做一些通用的改写,例如各种 push down,形成 FlinkLogicalPlan。这部分优化是流批通用的。然后流和批根据各自底层的实现,进行特定的优化,例如批上会根据 Cost 选择不同的 Join 算...
Subqueries: Know what a subquery is and how to apply it. Nested Queries: A discussion of exactly what nested queries in SQL are all about. Correlated Subqueries: Learn how correlated subqueries work and when to use them. Common Use Cases: Think of some real-world examples of using subqueries...
Look at an example of a subquery, which is a query that is nested in a SELECT, INSERT, UPDATE, or DELETE statement, or inside another subquery in SQL Server.
limit_with_subquery.sql SELECT * FROM ( SELECT * FROM books ORDER BY price DESC ) AS top_books LIMIT 3; The subquery sorts the books by price in descending order, and the outer query uses LIMIT 3 to return the top 3 results.
JOINS work faster in large datasets The performance of the subquery decreases as it has to be optimized Readability is hard in the case of complex queries It is easy to read since it simplifies the complex queries JOINS are executed quickly It executes slowly since the inner and outer query ...