Data merging skills in MySQL

In MySQL, sometimes data needs to be merged for better data analysis and mining. Here are some common MySQL data merging techniques.

1. UNION operation

UNION is one of the most commonly used merge techniques in MySQL. It is used to merge two or more SELECT query result sets into one result set and automatically remove duplicate records. The basic syntax of UNION is as follows:

SELECT 列1, 列2, 列3 FROM 表1
UNION
SELECT 列1, 列2, 列3 FROM 表2;

Among them, the UNION operator merges two query results into one result set and automatically removes duplicate rows. If you want to keep duplicate rows, you can use the UNION ALL operator.

2. JOIN operation

JOIN operation is another commonly used merging technique in MySQL. It is used to combine data from two or more tables into a result set, which can be related according to different conditions.

For example, we have an orders table to store order information and a customers table to store customer information. Now we want to merge the information in the orders table and customer table together to get the order information of each customer. You can use the following JOIN operation:

SELECT orders.order_id, customers.customer_name
FROM orders
JOIN customers
ON orders.customer_id = customers.customer_id;

Among them, JOIN and ON are keywords in the JOIN operation. The ON statement specifies the columns to be related. In this example, we are making a correlation based on the customer_id column.

3. Subquery

Subquery is another merging technique in MySQL. It contains one query statement within another query statement so that the results of the other query statement are used in the query statement. For example, we have an orders table to store order information and a products table to store product information. Now we want to query the sales volume and sales volume of each product. You can use the following subquery technique:

SELECT products.product_id, products.product_name,
(SELECT COUNT(*) FROM orders WHERE order_details.product_id = products.product_id) AS sales_count,
(SELECT SUM(order_details.price * order_details.quantity) FROM orders JOIN order_details ON orders.order_id = order_details.order_id WHERE order_details.product_id = products.product_id) AS sales_amount
FROM products;

In this example, we use two subqueries to calculate the sales volume and sales amount of each product. These two subqueries are used in the SELECT statement to calculate the quantity and amount of each product in the order table and order details table respectively.

Summary

The above are several commonly used data merging techniques in MySQL, including UNION operations, JOIN operations and subqueries. Using these techniques can simplify the data merging process and improve the efficiency of data analysis and mining.

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