本章主要为多表联查的相关操作。
关键词：left join、reght join、inner join、join
1. 左连接查询：
语法：select * from [PrimaryTable] left join [ForeignTable] on PrimaryTable.PrimarykeyField = ForeignTable.ForeignkeField where [Term];
示例：select * from class left join student on class.c_id = student.s_cid;
注释：left join ... on 的连表查询结果，只显示左侧表的全部信息，右侧表只显示可对应的数据，如对应不上的数据显示null。
2. 右连接查询：
语法：select * from [PrimaryTable] reght join [ForeignTable] on PrimaryTable.PrimarykeyField = ForeignTable.ForeignkeyField where [Term];
示例：select * from class reght join student on class.c_id = student.s_cid;
注释：reght join ... on 的连表查询结果，只显示右侧表的全部信息，左侧表只显示可对应数据，其余不显示。
3. 内连接查询：
语法：select * from [PrimaryTable] inner join [ForeignTable] on PrimaryTable.PrimarkeyField = ForeignTable.ForeignkeyField where [Term];
示例：select * from class inner join student on class.c_id = student.s_cid;
注释：inner join ... on 的作用和 join ... on 的作用相同，有匹配数据就显示。
4. 其它联表查询方式：
语法：select * from [TableName1], [TabelName2] where Table1.FieldName = Table2.FieldName and [Term];
示例：select * from class, student where class.c_id = student.s_cid;
注释：空。
5. 嵌套查询方式：
语法①：select * from [TableName1] where Table1.FieldName = (select Table2.FieldName from TableName2 where [Term]);
语法②：select * from [TableName1] where Table1.FieldName in (select Table2.FieldName from TableName2 where [Term]);
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