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# Chapter 34. Lookup Tables

Lookup tables are data structures that allow fast access to data stored using a known key or SQL query. This way you can reduce the need to browse database or data files.

| Warning
Remember that you should not use lookup tables in the `init()`, `preExecute()`, or `postExecute()` functions of CTL template and the same methods of Java interfaces.

All data records stored in any lookup table are kept in files, in databases or cached in memory.

Lookup tables can be internal, or external (shared).

- Internal: See Internal Lookup Tables.

  Internal lookup tables can be:

  - Externalized: See Externalizing Internal Lookup Tables.

  - Exported: See Exporting Internal Lookup Tables.

- External (shared): See External (Shared) Lookup Tables.

  External (shared) lookup tables can be:

  - Linked to the graph: See Linking External (Shared) Lookup Tables.

  - Internalized: See Internalizing External (Shared) Lookup Tables.

##### Types of Lookup Tables

- Simple lookup table: See Simple Lookup Table.

- Database lookup table: See Database Lookup Table.

- Range lookup table: See Range Lookup Table.

- Persistent lookup table: See Persistent Lookup Table.

- Aspell lookup table: See Aspell Lookup Table.

Lookup tables can be accessed using CTL functions, see Lookup Table Functions.

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