Different usage scenarios of subqueries as retrieval tables and the question of whether to add aliases

For more details, please refer to the [Subquery] subsection under [Data Analysis Advanced Road Column]

1, subquery

Subquery refers to a query in which a query statement is nested inside another query statement. This feature has been introduced since MySQL 4.1.The subquery is calculated first in the SELECT clause, and the result of the subquery is used as the filter condition of another query in the outer layer. The query can be based on one table or multiple tables.Commonly used operators in subqueries are ANY (SOME), ALL, IN, EXISTS.
Subqueries can be added to SELECT, UPDATE and DELETE statements, and can be nested multiple levels.Comparison operators such as "<", "<=", ">", ">=", and "!=" can also be used in subqueries.

The main points of its attention include the following:

1. The result of one query is used as the condition of another query
2. There is a nesting of queries, the inner query is called a subquery
3. Use parentheses for subqueries

Subqueries are mainly divided into the following three cases, the corresponding cases are as follows:
    1. A single row and a single column means that the result is one such as 1. In this case, no alias is required
  2. Multiple rows and single columnsThat is, the result is a set of data such as (1, 2, 3). In this case, no alias is required.
3. Multiple rows and multiple columns means the result is a table. In this case An alias is required to access fields in the table

2. The result of the subquery is a single row and a single column

Example: Query the product information with the highest price by means of sub-query

  # Through sub-query, query the product information with the highest price
  -- 1. First query the highest price
SELECT MAX(price) FROM products;
   -- 2. Use the highest price asConditions, get product information