Which one is more efficient between SQL neutron query and table connection as where filter condition?

 

In some cases, using subqueries is more efficient than using table joins. Using a subquery may be faster if the subquery can generate a small number of result sets. For example, when you need to retrieve certain rows from a table, you can use a subquery to generate a row number and use that row number in the WHERE clause in the main query. In this case, using a subquery is more efficient than using Table joins are higher.

In other cases, it may be more efficient to use table joins. If complex data manipulation between multiple tables is required, it may be faster to use table joins, especially when dealing with large amounts of data. Therefore, depending on the specific situation, you need to choose to use subquery or table join according to the actual data and query situation.

The WHERE clause of SQL is used to filter the result set and select rows that meet the conditions according to the specified conditions.

The basic syntax of the WHERE clause is as follows:

sql复制代码SELECT column1, column2, ...  
 FROM table_name  
 WHERE condition;

where ​column1, column2, ...​is the name of the column to select from the result set, ​table_name​is the name of the table from which data is to be selected, and ​condition​is the condition to filter on.

For example, suppose we have a table ​students​called , with columns ​id​, , ​name​and ​age​. If we want to select students whose age is greater than 18, we can use the following query:

sql复制代码SELECT * FROM students  
 WHERE age > 18;

In this query, we have used ​WHERE​the clause to filter out students whose age is less than or equal to 18.

In addition to the basic syntax, the WHERE clause has some other options, such as ​AND​and ​OR​, which can be used to combine multiple conditions. For example:

sql复制代码SELECT * FROM students  
 WHERE age > 18  
     OR name = 'John'  
     OR age = 30;

In this query, we have used ​OR​the keyword to combine two conditions and only if both conditions are met, the student will be selected.

A subquery is a query statement used in SQL to extract data that allows you to nest one query within another to select specific rows from a table. Subqueries can help you reduce query complexity and improve query performance.

A subquery is a SELECT statement that contains one or more FROM clauses, usually used to select specific rows from another table. A subquery can also contain a WHERE clause to filter the result set.

Here is an example of a simple subquery:

Suppose you have a table ​students​called , with columns ​id​, , ​name​and ​age​. You want to select students whose age is greater than 18, you can use the following query:

sql复制代码SELECT * FROM students  
 WHERE age > 18;

This query uses a subquery to select rows from ​age​the column that are older than 18. Note that ​WHERE​the clause to filter out rows with an age less than or equal to 18.

Subqueries allow you to select data from tables more easily and reduce the complexity of queries. However, when using subqueries, make sure they don't overcomplicate the query or return incorrect results. If you're not sure whether you should use subqueries, be sure to test carefully and read other users' query practices.