sql join语句语法_SQL Left Join语句：示例语法

sql join语句语法

对于本指南，我们将讨论SQL LEFT JOIN。 (For this guide we’ll discuss the SQL LEFT JOIN.)

Using the keyword LEFT before JOIN causes the system to start with the student (LEFT) table but will return NULL from the RIGHT table if there are no rows for the LEFT table student.

在JOIN之前使用关键字LEFT会导致系统从学生表(LEFT)开始，但是如果LEFT表Student没有行，它将从RIGHT表返回NULL。

Note that studentID 9 appears here but the data from the contact table is just shown as NULL.

请注意，此处显示了StudentID 9，但联系表中的数据仅显示为NULL。

SELECT a.studentID, a.FullName, a.programOfStudy,
b.`student-phone-cell`, b.`student-US-zipcode`
FROM student AS a
LEFT JOIN `student-contact-info` AS b ON a.studentID = b.studentID;
           

+-----------+------------------------+------------------+--------------------+--------------------+
| studentID | FullName               | programOfStudy   | student-phone-cell | student-US-zipcode |
+-----------+------------------------+------------------+--------------------+--------------------+
|         1 | Monique Davis          | Literature       | 555-555-5551       |              97111 |
|         2 | Teri Gutierrez         | Programming      | 555-555-5552       |              97112 |
|         3 | Spencer Pautier        | Programming      | 555-555-5553       |              97113 |
|         4 | Louis Ramsey           | Programming      | 555-555-5554       |              97114 |
|         5 | Alvin Greene           | Programming      | 555-555-5555       |              97115 |
|         6 | Sophie Freeman         | Programming      | 555-555-5556       |              97116 |
|         7 | Edgar Frank "Ted" Codd | Computer Science | 555-555-5557       |              97117 |
|         8 | Donald D. Chamberlin   | Computer Science | 555-555-5558       |              97118 |
|         9 | Raymond F. Boyce       | Computer Science | NULL               |               NULL |
+-----------+------------------------+------------------+--------------------+--------------------+
9 rows in set (0.00 sec)
           

完整的表格清单供参考 (Complete table listings for reference)

student or LEFT table SQL

学生表或左表SQL

SELECT a.studentID, a.FullName, sat_score, a.programOfStudy, schoolEmailAdr 
FROM student AS a;
           

student or LEFT table data

学生或左表数据

+-----------+------------------------+-----------+------------------+------------------------+
| studentID | FullName               | sat_score | programOfStudy   | schoolEmailAdr         |
+-----------+------------------------+-----------+------------------+------------------------+
|         1 | Monique Davis          |       400 | Literature       | [email protected] |
|         2 | Teri Gutierrez         |       800 | Programming      | [email protected]    |
|         3 | Spencer Pautier        |      1000 | Programming      | [email protected] |
|         4 | Louis Ramsey           |      1200 | Programming      | [email protected]   |
|         5 | Alvin Greene           |      1200 | Programming      | [email protected]   |
|         6 | Sophie Freeman         |      1200 | Programming      | [email protected]  |
|         7 | Edgar Frank "Ted" Codd |      2400 | Computer Science | [email protected]   |
|         8 | Donald D. Chamberlin   |      2400 | Computer Science | [email protected]  |
|         9 | Raymond F. Boyce       |      2400 | Computer Science | [email protected] |
+-----------+------------------------+-----------+------------------+------------------------+
9 rows in set (0.00 sec)

student contact or RIGHT table SQL	
```sql
select * from `student-contact-info` as b;
           

student contact or RIGHT table data

学生联络人或RIGHT表格资料

+-----------+----------------------------------+--------------------+--------------------+
| studentID | studentEmailAddr                 | student-phone-cell | student-US-zipcode |
+-----------+----------------------------------+--------------------+--------------------+
|         1 | [email protected]   | 555-555-5551       |              97111 |
|         2 | [email protected]  | 555-555-5552       |              97112 |
|         3 | [email protected] | 555-555-5553       |              97113 |
|         4 | [email protected]    | 555-555-5554       |              97114 |
|         5 | [email protected]     | 555-555-5555       |              97115 |
|         6 | [email protected]  | 555-555-5556       |              97116 |
|         7 | [email protected]    | 555-555-5557       |              97117 |
|         8 | [email protected]   | 555-555-5558       |              97118 |
+-----------+----------------------------------+--------------------+--------------------+
8 rows in set (0.00 sec)
           

As with all of these SQL things there is MUCH MORE to them than what’s in this introductory guide.

与所有这些SQL事物一样，它们比本入门指南中的内容要多得多。

I hope this at least gives you enough to get started.

我希望这至少能给您足够的入门。

Please see the manual for your database manager and have fun trying different options yourself.

请参阅数据库管理员的手册，并尝试自己尝试其他选项，这很有趣。

翻译自: https://www.freecodecamp.org/news/sql-left-join-commands-example-syntax/

sql join语句语法