A brief analysis of the syntax and common limitations of mysql in queries

MySQL is a very popular relational database management system. One of the most widely used languages ​​is SQL. We can implement various complex data operations through SQL statements. In MySQL, the in query is a very common query method. It can query the records in the specified column that match the query conditions (that is, the value after in). Compared with multiple consecutive OR queries with the same conditions, The in query is more concise and clear.

But in actual development, we often need to make some restrictions on the in query to avoid errors or improve query efficiency. Below, we'll cover some common in query limitations.

1. In query syntax and principle

Before we start to explain the limitations of in query, let’s first understand the syntax and principle of in query.

The syntax of in query is as follows:

SELECT column_name(s)
FROM table_name
WHERE column_name IN (value1, value2, ...);

For example, query the order information with goods numbers 1, 3, and 5 in the orders table:

SELECT * FROM orders WHERE goods_id IN (1,3,5);

inThe query principle is as follows :

• MySQL will open a hash table (Hash table) in memory to store all values ​​that meet the requirements.
• During the process of scanning the table, MySQL takes out each row in the table, extracts the values ​​of the columns that need to be used, and checks whether it is equal to any value in the hash table.
• If it meets the requirements, it will be added to the result set.

So, the efficiency of in query is affected by many factors such as the number of query conditions, the size of the memory hash table, and the number of records in the table. Next, we will explain the limitations of in queries.

2. In query limitations

1. Too many in query conditions

The more in query conditions, the greater the memory occupied by the MySQL hash table. The efficiency is slower. Therefore, in actual development, we should avoid using too many in query conditions. If there are too many query conditions, you can optimize the query by splitting multiple in query conditions and using subqueries.

For example, to query the order information with goods numbers 1, 3, 5, 7, and 9 in the orders table, you can write like this:

SELECT * FROM orders WHERE goods_id IN (1,3, 5) OR goods_id IN (7,9);

Or:

SELECT * FROM orders WHERE goods_id IN (SELECT id FROM goods WHERE id in (1,3,5,7,9 ));

In this way, we can split the query conditions to improve query efficiency.

2. Repeated in query conditions

In the in query, if the query conditions are repeated, the query efficiency will decrease and may cause duplicate data to appear in the result set. Therefore, when writing in query conditions, we should avoid repeated query conditions.

For example, to query the order information with goods numbers 1, 1, 3, 5, and 7 in the orders table, you can write like this:

SELECT * FROM orders WHERE goods_id IN (1,3, 5,7);

In this way, we can avoid repeated query conditions and improve query efficiency.

3. The in query condition is empty

If the in query condition does not contain any value, all data in the table will be queried. This will lead to reduced query efficiency and unnecessary data may appear. Therefore, when writing in query conditions, we should avoid empty query conditions.

For example, to query the order information with goods numbers 1, 3, 5, and 7 in the orders table, you can write:

SELECT * FROM orders WHERE goods_id IN (1,3,5, 7);

In this way, we can avoid the query condition being empty and improve query efficiency.

4. The in query condition type does not match

When performing an in query, the type of the query condition must match the type of the column in the table, otherwise the query will fail. Therefore, when writing in query conditions, we should ensure that the type of the query condition matches the type of the column in the table.

For example, to query the order information with goods numbers 1, 3, 5, and 7 in the orders table, you can write:

SELECT * FROM orders WHERE goods_id IN (1,3,5, 7);

In this way, we can ensure that the type of the query condition matches the type of the column in the table to avoid query failure.

5. Use in query and "not in" query at the same time

When in query and "not in" query are used at the same time, the query efficiency will be reduced. Therefore, in actual development, we should avoid using in queries and "not in" queries at the same time.

For example, to query the order information of goods number 1, 3, 5, and 7 in the orders table, but does not include the order information of goods number 9, you can write like this:

SELECT * FROM orders WHERE goods_id IN (1,3,5,7) AND goods_id NOT IN (9);

In this way, we can avoid using in query and "not in" query at the same time and improve query efficiency.

3. Summary

Through the introduction of this article, we understand the grammatical principles and common limitations of in query. In actual development, we should avoid using too many query conditions, avoid duplication of query conditions, avoid empty query conditions, ensure that the type of query conditions matches the type of columns in the table, and avoid using in queries and "not" at the same time according to specific circumstances. in" query to improve query efficiency and reduce the possibility of query errors.

The above is the detailed content of A brief analysis of the syntax and common limitations of mysql in queries. For more information, please follow other related articles on the PHP Chinese website!