SQL LEFT JOIN 关键字用来选择所有在左表中有匹配的记录，而不管在右表中是否有匹配。
SQL LEFT JOIN 语法
SELECT Table1.Column1, Table1.Column2, ... , Table2.Column1, Table2.Column2, ... FROM Table1 LEFT JOIN Table2 ON Table1.ColumnName = Table2.ColumnName
SQL LEFT JOIN 范例
数据表： Employees
| EmployeeId | FirstName | LastName | Department | Salary |
|---|---|---|---|---|
| 203 | Mazojys | Fxoj | Finance | 78000 |
| 204 | Jozzh | Lnanyo | Finance | 45800 |
| 205 | Syllauu | Dfaafk | Finance | 57000 |
| 206 | Gecrrcc | Srlkrt | Finance | 62000 |
数据表： Tickets
| TicketID | TicketNo | EmployeeId |
|---|---|---|
| 1 | 2349 | 203 |
| 2 | 2400 | 204 |
| 3 | 2438 | 205 |
选择所有的职员(Employees)，如果它们有Tickets，列出Tickets：
SELECT Employees.EmployeeId
, Employees.FirstName
, Employees.LastName
, Tickets.TicketNo
FROM Employees
LEFT JOIN Tickets
ON Employees.EmployeeId = Tickets.EmployeeId
结果会类似于：
| EmployeeId | FirstName | LastName | TicketNo |
|---|---|---|---|
| 203 | Mazojys | Fxoj | 2349 |
| 204 | Jozzh | Lnanyo | 2400 |
| 205 | Syllauu | Dfaafk | 2438 |
| 206 | Gecrrcc | Srlkrt |
注：EmployeeID为206的职员在Tickets表中没有任何匹配，不过他仍然出现在结果集中，TicketNo值为null。
Nice article about left join with good example); good to see such kinds of article
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It’s amazing how many programmers dont know their joins. I’d say 99% never use anything more than an inner join
I agree left / right joins are very important but seldom used. Not relevant in some situation but when they are called for they do a much better job than inner joins. I guess people simply don’t know that they exist, and how they work.
Nice tutorial.
I use left joins all the time. Couldn’t imagine “life” without them