what does union mean in sql

The UNION operator is used in SQL to merge rows with the same structure, producing a table containing all unique rows. It works by merging result sets, removing duplicate rows and returning remaining rows. Unlike UNION ALL, UNION returns only unique rows. When using UNION, note that both tables must have the same number of columns and data types.

UNION meaning in SQL

UNION is a SQL operator used to combine values ​​from two or Rows from multiple tables with the same structure (number of columns and data types). It creates a new table containing all unique rows that either exist in the first table or the second table.

How UNION works

The UNION operator works through the following steps:

1. Merge the result set: Will Rows from both tables are merged into a temporary result set.
2. Delete duplicate rows: Delete duplicate rows from the temporary result set, retaining only unique rows.
3. Return results: Return the remaining unique rows as a new table.

The difference between UNION and UNION ALL

The UNION ALL operator is similar to UNION, but it does not delete duplicate rows. UNION ALL returns all merged rows, including duplicate rows.

UNION syntax

The basic form of UNION syntax is as follows:

<code class="sql">SELECT 列名1, 列名2, ...
FROM 表名1
UNION
SELECT 列名1, 列名2, ...
FROM 表名2;</code>

For example, the following query uses the UNION operator to merge from two tables Data for students and teachers:

<code class="sql">SELECT id, name
FROM 学生
UNION
SELECT id, name
FROM 教师;</code>

This will create a new table containing the id and name of all students and the id and name of all teachers. Note that if the Student and Teacher tables have the same id value, they will only be listed once.

Notes on using UNION

When using UNION, you need to pay attention to the following:

• The two tables must have the same number of columns and data type.
• UNION returns only unique rows, which may affect the results.
• UNION ALL is used to return all rows, including duplicate rows.

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