67 lines
1.8 KiB
Markdown
67 lines
1.8 KiB
Markdown
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---
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title: SQL Union Operator
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localeTitle: SQL联盟运营商
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---
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## SQL联盟运营商
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### 描述
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对于本指南，我们将讨论SQL语句的UNION运算符部分。
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UNION运算符用于将多个select语句的结果组合到一个结果集中。
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SQL语句的Select语句中的列数必须相同。
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### 基本例子
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SQL语句
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```sql
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SELECT 'aaaaa'
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UNION
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SELECT 'bbbbbbbbb';
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```
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产量
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```text
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+-----------+
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| aaaaa |
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+-----------+
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| aaaaa |
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| bbbbbbbbb |
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+-----------+
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2 rows in set (0.00 sec)
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```
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### 使用学生表的示例
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SQL语句
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```sql
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SELECT StudentID, FullName FROM student WHERE studentID BETWEEN 1 AND 5
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UNION
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SELECT studentID, studentEmailAddr FROM `student-contact-info` WHERE studentID BETWEEN 7 AND 8;
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```
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产量
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\`\`\`文字 + ----------- + -------------------------------- + | StudentID | FullName | + ----------- + -------------------------------- + | 1 |莫妮克戴维斯| | 2 | Teri Gutierrez | | 3 | Spencer Pautier | | 4 |路易斯拉姆齐| | 5 | Alvin Greene | | 7 | Maximo.Smith@freeCodeCamp.org | | 8 | Michael.Roach@freeCodeCamp.ort | + ----------- + -------------------------------- + 7行（0.00秒）
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```
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## SQL UNION ALL Operator
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The UNION ALL operator is an extension to UNION operator where it should result you a A+B of rows in the ouptput assuming A and B is your input, in simple terms UNION ALL doesn't deduplicate.
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### Basic Syntax
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SQL Statement
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```
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SQL SELECT expression1，expression2，... expression _n FROM表 \[条件\] UNION ALL SELECT expression1，expression2，... expression_ n FROM表 \[条件\]; \`\`\`
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与所有这些SQL事物一样，它们比本入门指南中的内容更多。
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我希望这至少足以让你开始。
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请参阅您的数据库管理员手册，并自己尝试不同的选项。
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