Database structure

Types of tables

The database tables used by TYPO3 can be roughly divided into two categories:

Internal tables

Tables that are used internally by the system and are invisible to backend users (for example, be_sessions, sys_registry, cache-related tables). In the Core extension, there are often dedicated PHP APIs for managing entries in these tables, for instance, the caching framework API.

Managed tables

Tables that can be managed via the TYPO3 backend are shown in the Web > List module and can be edited using the FormEngine.

Requirements

There are certain requirements for such managed tables:

The table must be configured in the global TCA array, for example:
- table name
- features that are required
- fields of the table and how they should be rendered in the backend
- relations to other tables
and so on.
The table must contain at least these fields:
- uid - an auto-incremented integer and primary key for the table, containing the unique ID of the record in the table.
- pid - an integer pointing to the uid of the page (record from pages table) to which the record belongs.
The fields are created automatically when the table is associated with a TCA configuration.

Typical fields

A title field holding the title of the record as seen in the backend.
A description field holding a description displayed in the Web > List view.
A crdate field holding the creation time of the record.
A tstamp field holding the last modification time of the record.
A sorting field holding an order when records are sorted manually.
A deleted field which tells TYPO3 that the record is deleted (actually implementing a "soft delete" feature; records with a deleted field are not truly deleted from the database).
A hidden or disabled field for records which exist but should not be used (for example, disabled backend users, content not visible in the frontend).

Note

With the exception of the uid and pid fields, all other fields do not automatically fill a role as soon as they exist. Their existence must be declared in the TCA configuration. This means that such fields can also be named freely, the above are the default names TYPO3 uses - for consistency reasons it is recommended to name them that way.

The "pages" table

The pages table has a special status: It is the backbone of TYPO3, as it provides the hierarchical page structure into which all other records managed by TYPO3 are placed. All other managed tables in TYPO3 have a pid field that points to a uid record in this table. Thus, each managed table record in TYPO3 is always placed on exactly one page in the page tree. This makes the pages table the mother of all other managed tables. It can be seen as a directory tree with all other table records as files.

Standard pages are literally website pages in the frontend. But they can also be storage spaces in the backend, similar to folders on a hard disk. For each record, the pid field contains a reference to the page where that record is stored. For pages, the pid fields behaves as a reference to their parent pages.

The special "root" page has some unique properties: its pid is 0 (zero), it does not exist as a row in the pages table, only users with administrative rights can access records on it, and these records must be explicitly configured to reside in the root page - usually, table records can only be created on a real page.

MM relations

When tables are connected via a many-to-many relationship, another table must store these relations. Examples are the table storing relations between categories and categorized records (sys_category_record_mm) or the table storing relations between files and their various usages in pages, content elements, etc. (sys_file_reference). The latter is an interesting example, because it actually appears in the backend, although only as part of inline records.

Other tables

The internal tables which are not managed through the TYPO3 backend serve various purposes. Some of the most common are:

Cache: If a cache is defined to use the database as a cache backend, TYPO3 automatically creates and manages the relevant cache tables.
System information: There are tables that store information about sessions, both frontend and backend (fe_sessions and be_sessions respectively), a table for a central registry (sys_registry) and some others.

All these tables are not subject to the uid/pid constraint mentioned above, but they may have such fields if it is convenient for some reason.

There is no way to manage such tables through the TYPO3 backend, unless a specific module provides some form of access to them. For example, the System > Log module provides an interface for browsing records from the sys_log table.