oracle query foreign key

In database design, foreign keys can be used to achieve association and referential integrity between data tables to ensure data consistency and correctness. A foreign key means that a field value or a group of field values ​​in one table must be a reference to a field value or a group of field values ​​in another table. When querying foreign keys in a relational database, you can use SQL statements and specific Oracle commands to obtain relevant information to better understand and manage the database. Below we will introduce in detail how to query foreign keys in the Oracle database.

1. SQL Query Foreign Keys

To query the information of foreign keys in the Oracle database, you can use SQL statements. The following system tables are mainly used:

1, ALL_CONSTRAINTS

ALL_CONSTRAINTS is a system table that contains all constraints in the Oracle database. Use this table to query the foreign key information of all tables in the database.

Example statement:

SELECT * FROM ALL_CONSTRAINTS WHERE CONSTRAINT_TYPE='R' AND OWNER='SCOTT' ORDER BY TABLE_NAME, CONSTRAINT_NAME;

Among them, OWNER='SCOTT' means querying the foreign key of the SCOTT user, and CONSTRAINT_TYPE='R' means querying the foreign key constraints. Query results are sorted according to TABLE_NAME and CONSTRAINT_NAME.

2. ALL_CONS_COLUMNS

ALL_CONS_COLUMNS is a system table containing constraint columns in the Oracle database. Use this table to query column information for each foreign key in the table.

Example statement:

SELECT * FROM ALL_CONS_COLUMNS WHERE OWNER='SCOTT' AND TABLE_NAME = 'DEPT' ORDER BY CONSTRAINT_NAME, POSITION;

Among them, OWNER='SCOTT' means querying the SCOTT user's table, and TABLE_NAME = 'DEPT' means querying the foreign key column information of the DEPT table. Query results are sorted according to CONSTRAINT_NAME and POSITION.

In addition, ALL_TAB_COLUMNS can query detailed information such as column names and data types of specific tables.

Example statement:

SELECT COLUMN_NAME, DATA_TYPE FROM ALL_TAB_COLUMNS WHERE OWNER='SCOTT' AND TABLE_NAME='DEPT';

Among them, OWNER='SCOTT' means querying the SCOTT user's table, and TABLE_NAME='DEPT' means querying the column information of the DEPT table. Query results include column names and data types.

2. Oracle commands to query foreign keys

Oracle commands can also be used to query foreign key information. Mainly use the following three commands:

1, DESCRIBE

The DESCRIBE command is used to query the structural information of the table in the Oracle database, including table name, column name, data type, length, null value/ Non-null values, etc.

Example command:

DESCRIBE DEPT;

This command is used to query the structural information of the DEPT table.

2. ALTER TABLE

The ALTER TABLE command is used to change the structural information of tables in the Oracle database, including adding, modifying, deleting tables, columns, constraints and other operations.

Example command:

ALTER TABLE EMP ADD CONSTRAINT FK_EMP_DEPT FOREIGN KEY(DEPTNO) REFERENCES DEPT(DEPTNO);

This command is used to add a foreign key constraint named FK_EMP_DEPT to the EMP table and associate the DEPTNO field of the DEPT table.

3. SHOW CONSTRAINT

The SHOW CONSTRAINTS command is used to display all constraint information of tables in the Oracle database, including primary keys, foreign keys, checks, uniques, etc.

Example command:

SHOW CONSTRAINTS EMP;

This command is used to query all constraint information in the EMP table, including primary keys and foreign keys.

Summary

The above is how to use SQL statements and Oracle commands to query foreign key information in the Oracle database. Querying foreign key constraints can help us better understand the relationships between database tables, and can also be used to detect and repair association errors between tables. Finally, if you want to learn more about the use of Oracle database and the principles of database design and management, please continue to study related courses.

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