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• ¤ÇÒÁÊÑÁ¾Ñ¹¸ì¢Í§ËÅÒÂä»ÂѧËÅÒÂ
• ¤ÇÒÁÊÑÁ¾Ñ¹¸ì one-To-One


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• ÇÔ¸Õ¡ÒáÓ˹´¤ÇÒÁÊÑÁ¾Ñ¹¸ìẺ˹Öè§ä»ÂѧËÅÒ ËÃ×Í One-To-One
• ÇÔ¸Õ¡ÒáÓ˹´ËÅÒÂàÁ×è͵éͧ¡ÒÃËÅÒ¤ÇÒÁÊÑÁ¾Ñ¹¸ì


• ¤ÇÒÁÊÍ´¤Åéͧ¢Í§ referential
• ¡ÒûÃѺ»Ãا cascading áÅÐź
• ª¹Ô´¡ÒÃà¢éÒÃèÇÁ

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¤ÇÒÁÊÑÁ¾Ñ¹¸ì one-To-One

In a one-to-one relationship, a row in table A can have no more than one matching row in table B, and vice versa. A one-to-one relationship is created if both of the related columns are primary keys or have unique constraints.

This type of relationship is not common because most information related in this way would be all in one table. You might use a one-to-one relationship to:

• Divide a table with many columns.
• Isolate part of a table for security reasons.
• Store data that is short-lived and could be easily deleted by simply deleting the table.
• Store information that applies only to a subset of the main table.

In Access, the primary key side of a one-to-one relationship is denoted by a key symbol. The foreign key side is also denoted by a key symbol.

How to Define Relationships Between Tables

When you create a relationship between tables, the related fields do not have to have the same names. However, related fields must have the same data type unless the primary key field is an AutoNumber field. You can match an AutoNumber field with a Number field only if thefieldSizeproperty of both of the matching fields is the same. For example, you can match an AutoNumber field and a Number field if thefieldSizeproperty of both fields is Long Integer. Even when both matching fields are Number fields, they must have the samefieldSizeproperty setting.

How to Define a One-To-Many or One-To-One Relationships

To create a one-to-many or a one-to-one relationship, follow these steps:

1. Close any tables that you have open. You cannot create or modify relationships between open tables.
2. In Access 2002 or in Access 2003, follow these steps:
   1. Press F11 to switch to the Database window.
   2. 㹡ÒÃà¤Ã×èͧÁ×ÍàÁ¹Ù ¤ÅÔ¡¤ÇÒÁÊÑÁ¾Ñ¹¸ì.
   㹡ÒÃà¢éÒ¶Ö§ 2007 ¤ÅÔ¡¤ÇÒÁÊÑÁ¾Ñ¹¸ì㹡ÒÃáÊ´§/«è͹¨Ñ´¡ÅØèÁ㹡ÒÃDatabase Toolsá·çº
3. If you have not yet defined any relationships in your database, theáÊ´§µÒÃÒ§¡ÅèͧâµéµÍº¨Ð»ÃÒ¡¯¢Öé¹â´ÂÍѵâ¹ÁÑµÔ If you want to add the tables that you want to relate, but theáÊ´§µÒÃÒ§dialog box is not displayed, clickáÊ´§µÒÃҧ㹡ÒäÇÒÁÊÑÁ¾Ñ¹¸ìàÁ¹Ù
4. Double-click the names of the tables that you want to relate, and then close theáÊ´§µÒÃÒ§¡ÅèͧâµéµÍº To create a relationship between a table and itself, add that table twice.
5. Drag the field that you want to relate from one table to the related field in the other table. To drag multiple fields, press CTRL, click each field, and then drag them.
   
   In most cases, you drag the primary key field (which is displayed in bold text) from one table to a similar field (often with the same name) called the foreign key in the other table.
6. ¡Ãкǹ¡ÒÃá¡é䢤ÇÒÁÊÑÁ¾Ñ¹¸ì¡ÅèͧâµéµÍº¨Ð»ÃÒ¡¯¢Öé¹ Ensure that the field names displayed in the two columns are correct. You can change them if necessary.
   
   Set the relationship options if necessary. If you need information about a specific item in theá¡é䢤ÇÒÁÊÑÁ¾Ñ¹¸ìdialog box, click the question mark button, and then click the item. These options will be explained in detail later in this article.
7. ¤ÅÔ¡ÊÃéÒ§to create the relationship.
8. Repeat steps 5 through 8 for each pair of tables that you want to relate.
   
   When you close theá¡é䢤ÇÒÁÊÑÁ¾Ñ¹¸ìdialog box, Microsoft Access asks if you want to save the layout. Whether you save the layout or not, the relationships that you create are saved in the database.
   
   ËÁÒÂà˵Ø::You can create relationships in queries as well as tables. However, referential integrity is not enforced with queries.

How to Define a Many-To-Many Relationships

To create a many-to-many relationship, follow these steps:

1. Create the two tables that will have a many-to-many relationship.
2. ÊÃéÒ§µÒÃÒ§ÊÒÁ àÃÕ¡¡ÒÃàª×èÍÁµè͵ÒÃÒ§ áÅéÇ à¾ÔèÁ¡ÒÃàª×èÍÁµè͵ÒÃÒ§ãËÁè¿ÔÅ´ìµèÒ§ æ ´éÇ¢éÍ¡Ó˹´à´ÕÂǡѹà»ç¹à¢µ¢éÍÁÙŤÕÂìËÅÑ¡¨Ò¡áµèÅÐÊͧµÒÃÒ§Í×è¹ æ ã¹µÒÃÒ§àª×èÍÁµèÍ ¿ÔÅ´ì¤ÕÂìËÅÑ¡·Ó§Ò¹à»ç¹¤ÕÂìµèÒ§»ÃÐà·È ¤Ø³ÊÒÁÒöà¾ÔèÁࢵ¢éÍÁÙÅÍ×è¹ æ ä»ÂѧµÒÃÒ§àª×èÍÁµèÍ àªè¹à´ÕÂǡѺ·Õè¤Ø³ÊÒÁÒö价ÕèµÒÃÒ§Í×è¹
3. ã¹µÒÃÒ§àª×èÍÁµèÍ ¡Ó˹´¤ÕÂìËÅÑ¡¨ÐÃÇÁࢵ¢éÍÁÙŤÕÂìËÅÑ¡¨Ò¡Í×è¹ÊͧµÒÃÒ§ µÑÇÍÂèÒ§àªè¹ ã¹ TitleAuthors ¡ÒÃàª×èÍÁµè͵ÒÃÒ§ ¤ÕÂìËÅÑ¡¨Ð¶Ù¡·Ó¢Öé¹ OrderID áÅÐ ProductID ¿ÔÅ´ì
   
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   2. àÅ×͡ࢵ¢éÍÁÙÅËÃ×Íࢵ¢éÍÁÙÅ·Õè¤Ø³µéͧ¡ÒáÓ˹´à»ç¹¤ÕÂìËÅÑ¡ àÁ×è͵éͧ¡ÒÃàÅ×Í¡¿ÔÅ´ì˹Ö觿ÔÅ´ì ¤ÅÔ¡µÑÇàÅ×Í¡á¶ÇÊÓËÃѺࢵ¢éÍÁÙÅ·Õèµéͧ¡ÒÃ
      
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      㹡ÒÃà¢éÒ¶Ö§ 2007 ¤ÅÔ¡¤ÕÂìËÅѡ㹡ÒÃà¤Ã×èͧÁ×ͨѴ¡ÅØèÁ㹡ÒÃÍ͡Ẻá·çº
      
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4. ¡Ó˹´¤ÇÒÁÊÑÁ¾Ñ¹¸ìẺ˹Öè§ä»ÂѧËÅÒÂÃÐËÇèÒ§áµèÅеÒÃÒ§ËÅÑ¡·ÕèÊͧáÅеÒÃÒ§àª×èÍÁµèÍ

¤ÇÒÁÊÍ´¤Åéͧ¢Í§ referential

¤ÇÒÁÊÁºÙóì¢Í§ referential ¤×Í Ãкº¢Í§¡®·Õèãªé Microsoft Access à¾×èÍãËéá¹èã¨ÇèÒ ¤ÇÒÁÊÑÁ¾Ñ¹¸ìÃÐËÇèÒ§ÃÐàºÕ¹㹵ÒÃÒ§·Õèà¡ÕèÂÇ¢éͧäÁè¶Ù¡µéͧ áÅÐãËé¤Ø³â´ÂäÁèµÑé§ã¨äÁèź ËÃ×Íà»ÅÕè¹á»Å§¢éÍÁÙÅ·Õèà¡ÕèÂÇ¢éͧ ¤Ø³ÊÒÁÒöµÑ駤èÒ¤ÇÒÁÊÁºÙóì¢Í§ referential àÁ×èÍà§×è͹ä¢ã´ æ µèÍ仹Õé·Ñé§ËÁ´µÒÁ:

• ¿ÔÅ´ì·ÕèµÃ§¡Ñ¹¨Ò¡µÒÃÒ§ËÅÑ¡à»ç¹¤ÕÂìËÅÑ¡ ËÃ×ÍÁմѪ¹Õ·ÕèäÁè«éӡѹ
• ࢵ¢éÍÁÙÅà¡ÕèÂÇ¢éͧÁÕ¾ÔÁ¾ì¢éÍÁÙÅà´ÕÂǡѹ ÁÕ¢éÍ¡àÇé¹·ÕèÊͧ ÁÕ¿ÔÅ´ì AutoNumber ÊÒÁÒö¨Ðà¡ÕèÂÇ¢éͧ¡Ñºà¢µ¢éÍÁÙÅËÁÒÂàÅ¢·ÕèÁÕ¡ÒÃfieldSize¡ÒõÑ駤èҤسÊÁºÑµÔ¢Í§¨Ó¹Ç¹àµçÁÂÒÇ áÅÐà¾ÔèÁ¿ÔÅ´ì AutoNumber ´éÇÂfieldSize¡ÒõÑ駤èҤسÊÁºÑµÔ¢Í§ ID ¡ÒèÓÅͧẺÊÒÁÒö¨Ðà¡ÕèÂÇ¢éͧ¡Ñºà¢µ¢éÍÁÙÅËÁÒÂàÅ¢·ÕèÁÕ¡ÒÃfieldSize¡ÒõÑ駤èҤسÊÁºÑµÔ¢Í§¡ÒèÓÅͧẺ ID.
• µÒÃÒ§·Ñé§Êͧà»ç¹¢Í§°Ò¹¢éÍÁÙÅ Microsoft Access à´ÕÂǡѹ ¶éÒ µÒÃÒ§·Õèàª×èÍÁ⧵ÒÃÒ§ ¾Ç¡à¢Òµéͧà»ç¹µÒÃҧ㹠Microsoft Access ÃÙ» áÅФسµéͧà»Ô´°Ò¹¢éÍÁÙÅ·Õè¨Ðà¡çº¡ÒõÑ駤èÒ¤ÇÒÁÊÁºÙóì¢Í§ referential ¤ÇÒÁÊÁºÙóì¢Í§ referential äÁè¶Ù¡ºÑ§¤ÑºãªéÊÓËÃѺµÒÃÒ§·Õèàª×èÍÁ⧨ҡ°Ò¹¢éÍÁÙÅã¹ÃٻẺÍ×è¹ æ

¡®µèÍ仹Õé¹Óä»ãªéàÁ×èͤسãªé¤ÇÒÁÊÁºÙóì¢Í§ referential:

• ¤Ø³äÁèÊÒÁÒö»é͹¤èÒ㹿ÔÅ´ì¤ÕÂìÀÒ¹͡¢Í§µÒÃÒ§à¡ÕèÂÇ¢éͧ·ÕèäÁèÁÕÍÂÙè㹤ÕÂìËÅÑ¡¢Í§µÒÃÒ§ËÅÑ¡ However, you can enter a Null value in the foreign key, specifying that the records are unrelated. For example, you cannot have an order that is assigned to a customer that does not exist, but you can have an order that is assigned to no one by entering a Null value in the CustomerID field.
• You cannot delete a record from a primary table if matching records exist in a related table. For example, you cannot delete an employee record from the Employees table if there are orders assigned to the employee in the Orders table.
• You cannot change a primary key value in the primary table, if that record has related records. For example, you cannot change an employee's ID in the Employees table if there are orders assigned to that employee in the Orders table.

¡ÒûÃѺ»Ãا cascading áÅÐź

For relationships in which referential integrity is enforced, you can specify whether you want Microsoft Access to automatically cascade update or cascade delete related records. If you set these options, delete and update operations that would normally be prevented by referential integrity rules are allowed. When you delete records or change primary key values in a primary table, Microsoft Access makes the necessary changes to related tables to preserve referential integrity.

¶éҤس¤ÅÔ¡à¾×èÍàÅ×Í¡¹Ñé¹Cascade Update Related Fieldscheck box when you define a relationship, any time that you change the primary key of a record in the primary table, Microsoft Access automatically updates the primary key to the new value in all related records. For example, if you change a customer's ID in the Customers table, the CustomerID field in the Orders table is automatically updated for every one of that customer's orders so that the relationship is not broken. Microsoft Access cascades updates without displaying any message.

ËÁÒÂà˵Ø::If the primary key in the primary table is an AutoNumber field, selecting theCascade Update Related Fieldscheck box will have no effect, because you cannot change the value in an AutoNumber field.

¶éҤسàÅ×Í¡¹Ñé¹àÃÕ§«é͹źÃÐàºÕ¹·Õèà¡ÕèÂÇ¢éͧcheck box when you define a relationship, any time that you delete records in the primary table, Microsoft Access automatically deletes related records in the related table. For example, if you delete a customer record from the Customers table, all the customer's orders are automatically deleted from the Orders table (this includes records in the Order Details table related to the Orders records). When you delete records from a form or datasheet with theàÃÕ§«é͹źÃÐàºÕ¹·Õèà¡ÕèÂÇ¢éͧcheck box selected, Microsoft Access warns you that related records may also be deleted. However, when you delete records using a delete query, Microsoft Access automatically deletes the records in related tables without displaying a warning.

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There are three join types, as follows:

µÑÇàÅ×Í¡·Õè 1defines an inner join. An inner join is a join where records from two tables are combined in a query's results only if values in the joined fields meet a specified condition. In a query, the default join is an inner join that selects records only if values in the joined fields match.

µÑÇàÅ×Í¡ 2defines a left outer join. A left outer join is a join in which all the records from the left side of the LEFT JOIN operation in the query's SQL statement are added to the query's results, even if there are no matching values in the joined field from the table on the right.

Option 3defines a right outer join. A right outer join is a join in which all the records from the right side of the RIGHT JOIN operation in the query's SQL statement are added to the query's results, even if there are no matching values in the joined field from the table on the left.