MySQL之关联表查询技巧!（mysql关联表查询）

MySQL database is a popular database management system which enables businesses to effectively store and query data . A common database task is to select data from multiple tables based on relationships between them. To achieve this, we need to use join query.

Let’s take an example. There are two tables. The first one is a list of orders, with columns for order_id, item_id, item_name, and quantity. The second one is a list of prices for items, with columns for item_id and price. We need to select order_id, item_name, quantity, and price from both tables.

The query we need to write to achieve this begins with the SELECT command. We’ll want to select the order_id, item_name, quantity, and price columns from both tables, so we’ll write:

“`SELECT orders.order_id, orders.item_name, orders.quantity,

prices.price

FROM orders

INNER JOIN prices ON orders.item_id = prices.item_id;“`

This query gives us the desired columns from both tables as a result. The INNER JOIN clause combines data from both tables, based on the condition that the item_id from the orders table matches the item_id from the prices table.

We can also use LEFT JOIN and RIGHT JOIN clauses together with a WHERE condition to get our desired output.

“`SELECT orders.order_id, orders.item_name, orders.quantity,

prices.price

FROM orders

LEFT JOIN prices ON orders.item_id = prices.item_id

WHERE orders.quantity > prices.price;“`

Using LEFT JOIN and RIGHT JOIN has the advantage of allowing us to compare values between tables and to filter our results based on those comparisons. In this example, the query will select orders where the quantity is greater than the price.

In conclusion, joining tables is an essential skill when working with databases. By understanding how to use the SELECT, INNER JOIN, LEFT JOIN, and RIGHT JOIN clauses, we can effectively write queries that can join data from multiple tables.