What does ll mean in sql

LL in SQL stands for list of lists, which is used to combine the results of multiple subqueries or derived tables into a single result set in the SELECT statement to improve code readability, performance and modularity .

The meaning of LL in SQL

In SQL, LL represents the column list (list of lists). It refers to the results of a subquery or derived table enclosed in parentheses when returning a result set in a SELECT statement.

How to use LL

LL is typically used to combine multiple query results into a single result set. LL can be used in the following way:

<code class="sql">SELECT * FROM (
  SELECT column1 FROM table1
  UNION
  SELECT column2 FROM table2
) AS ll;</code>

The above query will return the combined result set of column1 and column2 from table1 and table2.

Benefits of LL

Using LL can bring the following benefits:

• Improve readability:Pass Splitting complex queries into multiple smaller queries can improve code readability.
• Improve performance: For some queries, using LL can optimize query performance.
• Achieving modularity: LL can decompose queries into reusable components to achieve modularity.

Example

The following example illustrates how to use LL:

<code class="sql">-- 计算总销售额
SELECT SUM(sales) AS total_sales
FROM (
  SELECT SUM(sales) AS sales
  FROM orders
  GROUP BY customer_id
);</code>

This query will calculate the total sales of all customers and The result is returned in the form of LL.

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