49 lines
3.9 KiB
Markdown
49 lines
3.9 KiB
Markdown
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---
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title: SQL Left Join
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localeTitle: SQL Left Join
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---
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## SQL Left Join
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### 使用示例
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对于本指南，我们将讨论SQL LEFT JOIN。
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### 左加入
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在JOIN之前使用关键字LEFT会导致系统以学生（LEFT）表开始，但如果LEFT表学生没有行，则会从RIGHT表返回NULL。
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请注意，studentID 9出现在此处，但联系表中的数据仅显示为NULL。
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```sql
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SELECT a.studentID, a.FullName, a.programOfStudy,
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b.`student-phone-cell`, b.`student-US-zipcode`
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FROM student AS a
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LEFT JOIN `student-contact-info` AS b ON a.studentID = b.studentID;
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```
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\`\`\`文字 + ----------- + ------------------------ + ------------ ------ + -------------------- + -------------------- + | studentID | FullName | programOfStudy |学生电话| student-US-zipcode | + ----------- + ------------------------ + ------------ ------ + -------------------- + -------------------- + | 1 |莫妮克戴维斯|文学| 555-555-5551 | 97111 | | 2 | Teri Gutierrez |编程| 555-555-5552 | 97112 | | 3 | Spencer Pautier |编程| 555-555-5553 | 97113 | | 4 |路易斯拉姆齐|编程| 555-555-5554 | 97114 | | 5 | Alvin Greene |编程| 555-555-5555 | 97115 | | 6 |索菲弗里曼|编程| 555-555-5556 | 97116 | | 7 |埃德加弗兰克“特德”科德|计算机科学| 555-555-5557 | 97117 | | 8 |唐纳德D. Chamberlin |计算机科学| 555-555-5558 | 97118 | | 9 |雷蒙德F.博伊斯|计算机科学| NULL | NULL | + ----------- + ------------------------ + ------------ ------ + -------------------- + -------------------- + 9行（0.00秒）
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```
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### Complete table listings for reference
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student or LEFT table SQL
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```
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SQL SELECT a.studentID，a.FullName，sat\_score，a.programOfStudy，schoolEmailAdr 来自学生AS;
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```
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student or LEFT table data
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```
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文本 + ----------- + ------------------------ + ----------- + ------------------ + ------------------------ + | studentID | FullName | sat\_score | programOfStudy | schoolEmailAdr | + ----------- + ------------------------ + ----------- + ------------------ + ------------------------ + | 1 |莫妮克戴维斯| 400 |文学| Monique@someSchool.edu | | 2 | Teri Gutierrez | 800 |编程| Teri@someSchool.edu | | 3 | Spencer Pautier | 1000 |编程| Spencer@someSchool.edu | | 4 |路易斯拉姆齐| 1200 |编程| Louis@someSchool.edu | | 5 | Alvin Greene | 1200 |编程| Alvin@someSchool.edu | | 6 |索菲弗里曼| 1200 |编程| Sophie@someSchool.edu | | 7 |埃德加弗兰克“特德”科德| 2400 |计算机科学| Edgar@someSchool.edu | | 8 |唐纳德D. Chamberlin | 2400 |计算机科学| Donald@someSchool.edu | | 9 |雷蒙德F.博伊斯| 2400 |计算机科学| Raymond@someSchool.edu | + ----------- + ------------------------ + ----------- + ------------------ + ------------------------ + 9行（0.00秒）
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学生联系或右表SQL
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```sql
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select * from `student-contact-info` as b;
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```
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学生联系或正确的表格数据 `text +-----------+----------------------------------+--------------------+--------------------+ | studentID | studentEmailAddr | student-phone-cell | student-US-zipcode | +-----------+----------------------------------+--------------------+--------------------+ | 1 | Monique.Davis@freeCodeCamp.org | 555-555-5551 | 97111 | | 2 | Teri.Gutierrez@freeCodeCamp.org | 555-555-5552 | 97112 | | 3 | Spencer.Pautier@freeCodeCamp.org | 555-555-5553 | 97113 | | 4 | Louis.Ramsey@freeCodeCamp.org | 555-555-5554 | 97114 | | 5 | Alvin.Green@freeCodeCamp.org | 555-555-5555 | 97115 | | 6 | Sophie.Freeman@freeCodeCamp.org | 555-555-5556 | 97116 | | 7 | Maximo.Smith@freeCodeCamp.org | 555-555-5557 | 97117 | | 8 | Michael.Roach@freeCodeCamp.ort | 555-555-5558 | 97118 | +-----------+----------------------------------+--------------------+--------------------+ 8 rows in set (0.00 sec)`
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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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