How to use SQL statements to perform data joins and union queries in MySQL?

How to use SQL statements to perform data connections and joint queries in MySQL?

Data joins and union queries are commonly used techniques in the SQL language to obtain and filter the required data in multiple tables. In MySQL, we can use the JOIN clause to implement data connections, and the UNION and UNION ALL clauses to implement joint queries of data. Next, we will introduce in detail how to use SQL statements to perform data connections and union queries in MySQL, and provide specific code examples.

1. Data connection:

Data connection is to associate data in multiple tables through common columns to obtain the required data. In MySQL, we can use four connection methods: INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN. Here are their detailed explanations and code examples:

• INNER JOIN (inner join): Only return rows that match both tables under the join condition.

SELECT 列名
FROM 表1
INNER JOIN 表2
ON 表1.列名 = 表2.列名;

• LEFT JOIN (left join): Returns all rows in the left table (that is, the first table), and rows in the right table that match the left table.

SELECT 列名
FROM 表1
LEFT JOIN 表2
ON 表1.列名 = 表2.列名;

• RIGHT JOIN (right join): Returns all rows in the right table (that is, the second table), and rows in the left table that match the right table.

SELECT 列名
FROM 表1
RIGHT JOIN 表2
ON 表1.列名 = 表2.列名;

• FULL JOIN (full join): Returns all rows in the left and right tables, regardless of matching or not.

SELECT 列名
FROM 表1
FULL JOIN 表2
ON 表1.列名 = 表2.列名;

2. Union query:

Union query is used to combine the result sets of multiple SELECT statements and return a temporary table containing all results. In MySQL, we can use UNION and UNION ALL to perform joint queries. Here are detailed explanations and code examples of them:

• UNION: Returns the result set of two or more SELECT statements, removing duplicate rows.

SELECT 列名
FROM 表1
WHERE 条件
UNION
SELECT 列名
FROM 表2
WHERE 条件;

• UNION ALL: Returns the result set of two or more SELECT statements that contains duplicate rows.

SELECT 列名
FROM 表1
WHERE 条件
UNION ALL
SELECT 列名
FROM 表2
WHERE 条件;

The above are the basic methods and code examples for using SQL statements to perform data connections and joint queries in MySQL. Through the flexible use of joins and union queries, you can easily obtain and filter the required data from multiple tables and meet different data analysis and query needs. I wish you good results when using SQL statements for data query!

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