Working with the Database tool window - Help | IntelliJ IDEA
Use this dialog to specify the settings for a relationship. Join Columns. Joined table columns for the relationship. Use icons general Reference: Persistence. Sep 8, I have a database created in MariaDB called persistence with two related tables employee user, create a project with IntelliJ IDEA. These DBCC CHECKCONSTRAINTS commands can be run as follows: DBCC CHECKCONSTRAINTS (TableName) - checks an individual.
When the insertion line appears, drop the field into position. The Lookup Wizard starts. Follow the instructions to complete the Lookup Wizard. The field appears in the table in Datasheet view. When you drag a field from an "other" unrelated table and then complete the Lookup Wizard, a new one-to-many relationship is automatically created between the table in the Field List and the table to which you dragged the field.
Edit a relationship You change a relationship by selecting it in the Relationships window and then editing it. Carefully position the cursor so that it points to the relationship line, and then click the line to select it. The relationship line appears thicker when it is selected. With the relationship line selected, double-click it. The Relationships window appears. If you have not yet defined any relationships and this is the first time you are opening the Relationships window, the Show Table dialog box appears.
If the dialog box appears, click Close. On the Design tab, in the Relationships group, click All Relationships. All tables with relationships are displayed, showing relationship lines.
Note that hidden tables tables for which the Hidden check box in the table's Properties dialog box is selected and their relationships will not be shown unless Show Hidden Objects is selected in the Navigation Options dialog box.
Click the relationship line for the relationship that you want to change. Double-click the relationship line. On the Design tab, in the Tools group, click Edit Relationships.
Make your changes, and then click OK. The Edit Relationships dialog box allows you to change a table relationship. Specifically, you can change the tables or queries on either side of the relationship, or the fields on either side.
You can also set the join type, or enforce referential integrity and choose a cascade option.
Advanced entity association mappings (Hibernate)
For more information about the join type and how to set it, see the section Set the join type. For more information about how to enforce referential integrity and choose a cascade option, see the section Enforce referential integrity.
Set the join type When you define a table relationship, the facts about the relationship inform your query designs. For example, if you define a relationship between two tables, and you then create a query that employs the two tables, Access automatically selects the default matching fields based upon the fields specified in the relationship.
You can override these initial default values in your query, but the values supplied by the relationship will often prove to be the correct ones.
Because matching and bringing together data from more than one table is something you will do frequently in all but the most simple databases, setting defaults by creating relationships can be time saving and beneficial. A multiple table query combines information from more than one table by matching the values in common fields. The operation that does the matching and combining is called a join.
Advanced entity association mappings (Hibernate)
Copying data from one table to another one Drag the source table to the destination table. In the dialog that opensspecify the data mapping info and other settings for the destination table. A separate file is created for each individual table or view.
Select the data source or the schemas, tables and views of interest. In the context menu, point to Dump Data to File s and select the output format e. In the dialog that opens, specify the destination directory and, if a single file is going to be created, the file name. Configuring data output formats and options To configure the output formats for the Dump Data to File s command see Saving data in files in various forms and formatsselect one of the following from the menu associated with the command: This command opens the CSV Formats dialog that lets you manage your delimiter-separated values formats e.
- Working with the Database tool window
- Create, edit or delete a relationship
- Add / Edit Relationship
Go to Scripts Directory. To map the association of shippingAddress in the User class as a shared primary key association, you also need the PrimaryKeyJoinColumn annotation: This is all that is needed to create a unidirectional one-to-one association on a shared primary key.
Note that you need PrimaryKeyJoinColumns plural instead if you map with composite primary keys. Hibernate has an extension annotation for custom identifier generators which you can use with the Address entity just like in XML: In many schemas, a to-one association is represented with a foreign key field and a unique constraint. One-to-one foreign key associations Instead of sharing a primary key, two rows can have a foreign key relationship. One table has a foreign key column that references the primary key of the associated table.
The source and target of this foreign key constraint can even be the same table: This is called a self-referencing relationship. The reason is simple: The database schema for this mapping is shown in figure 7. What mapping element can you add on the Address side to make this association bidirectional, so that access from Address to User is possible in the Java domain model?
You tell Hibernate that the user property of the Address class is the inverse of a property on the other side of the association. You can now call anAd-dress. There is no additional column or foreign key constraint; Hibernate manages this pointer for you. Should you make this association bidirectional? As always, the decision is up to you and depends on whether you need to navigate through your objects in that direction in your application code. If you call anAd-dress. We recommend that a foreign key-based one-to-one association, with a unique constraint on the foreign key column—is almost always best represented without a mapping on the other side.
Mapping a foreign key with annotations The JPA mapping annotations also support a one-to-one relationship between entities based on a foreign key column. The main difference compared to the mappings earlier in this topic is the use of JoinColumn instead of Prima-ryKeyJoinColumn. Hibernate will now enforce the multiplicity with the unique constraint.
If you want to make this association bidirectional, you need another OneToOne mapping in the Address class: The effect of the mappedBy attribute is the same as the property-ref in XML mapping: The last option we want to discuss is a bit more exotic: Imagine you have to model a data schema that represents an office allocation plan in a company. Common entities include people working at desks.
It seems reasonable that a desk may be vacant and have no person assigned to it. On the other hand, an employee may work at home, with the same result.
If you apply the mapping techniques we discussed in the previous sections, you may come to the following conclusions: The relationship is optional if the foreign key column is nullable.
In the current design, this table has only two columns: The multiplicity of these foreign key columns is enforced with a unique constraint on both—a particular person and desk can only be assigned once, and only one such an assignment can exist. Where does such an optional one-to-one relationship exist in CaveatEmptor? Sellers and buyers interact in CaveatEmptor by starting and bidding on auctions. The shipment of the goods seems to be outside the scope of the application; the seller and the buyer agree on a method of shipment and payment after the auction ends.
They can do this offline, outside of CaveatEmptor. On the other hand, you could offer an extra escrow service in CaveatEmptor. Sellers would use this service to create a trackable shipment once the auction completed. But you want more service in CaveatEmp-tor. This scenario calls for a Shipment entity with an optional one-to-one association to an Item. Look at the class diagram for this domain model in figure 7. A row in this table represents a Shipment made in the context of an auction.
The tables are shown in figure 7. You now map two classes to three tables: But if a row needs to be inserted because you called aShipment. You could map this association bidirectional, with the same technique on the other side.
Oracle Foreign Key Constraint: Enforcing Relationship Beween Tables
However, optional one-to-one associations are unidirectional most of the time. JPA also supports association join tables as secondary tables for an entity.
Mapping secondary join tables with annotations You can map an optional one-to-one association to an intermediate join table with annotations: The auction property mapping is a OneToOne; and as before, the foreign key column referencing the ITEM table is moved to the intermediate secondary table: The table for the target JoinColumn is named explicitly. Why would you use this approach instead of the simpler JoinTable strategy?
Declaring a secondary table for an entity is useful if not only one property the many-to-one in this case but several properties must be moved into the secondary table. This completes our discussion of one-to-one association mappings. To summarize, use a shared primary key association if one of the two entities seems more important and can act as the primary key source.
Use a foreign key association in all other cases, and a hidden intermediate join table when your one-to-one association is optional. We now focus on many-valued entity associations, including more options for one-to-many, and finally, many-to-many mappings.
Many-valued entity associations A many-valued entity association is by definition a collection of entity references. You mapped one of these in the previous topic, section 6.
One-to-many associations are the most important kind of entity association that involves a collection. A many-to-many association may always be represented as two many-to-one associations to an intervening class. This model is usually more easily extensible, so we tend not to use many-to-many associations in applications.
If you decide to map collections of entity references, there are a few options and more complex situations that we discuss now, including a many-to-many relationship. The many end of this association was implemented in Java with a Set; you had a collection of bids in the Item class. Why would you do this? To map a bidirectional one-to-many association as a bag, you have to replace the type of the bids collection in the Item persistent class with a Collection and an ArrayList implementation.
The mapping for the association between Item and Bid is left essentially unchanged: Even the tables are the same: Unidirectional and bidirectional lists If you need a real list to hold the position of the elements in a collection, you have to store that position in an additional column. Verify this with the table shown in figure 7. Also, because this mapping is unidirectional the collection is noninverseyou have to assume that there is no opposite side mapped to the same foreign key column where this constraint could be declared.
Whenever you have a noninverse collection of entity references most of the time a one-to-many with a list, map, or array and the foreign key join column in the target table is not nullable, you need to tell Hibernate about this.
Remember that Hibernate ignores the state of an inverse collection! This time, however, the collection contains information that is needed to update the database correctly: If you map a bidirectional one-to-many entity association with an indexed collection this is also true for maps and arraysyou have to switch the inverse sides. The collection becomes responsible for state synchronization, and the one side, the Bid, has to be made inverse.
Setting insert and update to false has the desired effect. As we discussed earlier, these two attributes used together make a property effectively read-only. This side of the association is therefore ignored for any write operations, and the state of the collection including the index of the elements is the relevant state when the in-memory state is synchronized with the database. The equivalent in JPA, an indexed collection in a bidirectional one-to-many mapping, is as follows: This mapping is noninverse because no mappedBy attribute is present.
We now discuss one more scenario with a one-to-many relationship: Optional one-to-many association with a join table A useful addition to the Item class is a buyer property. You can then call anItem.
If made bidirectional, this association will also help to render a screen that shows all auctions a particular user has won: From the point of view of the User class, the association is one-to-many.
The classes and their relationship are shown in figure 7. Why is this association different than the one between Item and Bid? The column has to be nullable—a particular Item may not have been bought as long as the auction is still running. We always try to avoid nullable columns in a relational database schema. Information that is unknown degrades the quality of the data you store. An optional entity association, be it one-to-one or one-to-many, is best represented in an SQL database with a join table.
You added a join table earlier in this topic, for a one-to-one association.