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+# Create a new version of the Catalogue Schema
+
+## Introduction
+
+The Catalogue schema is the description of the tables that represent the CTA Catalogue.
+
+The Catalogue schema is associated to a number that has the following format :
+```
+SCHEMA_VERSION_MAJOR.SCHEMA_VERSION_MINOR
+```
+*Example: 1.0, 1.1, 2.0*
+
+**WARNING**
+    The changing of the *SCHEMA_VERSION_MAJOR* number has to be modified **ONLY** if the changes made to the schema are [not backward-compatible](https://eoscta.docs.cern.ch/catalogue/upgrade/backward_incompatible_upgrades/) with the currently running version of CTA.
+
+**DANGER**
+    While starting, CTA will check the *SCHEMA_VERSION_MAJOR* schema version it is supposed to run against. If the *SCHEMA_VERSION_MAJOR* is not the correct one, CTA will not start.
+
+## Modify the Catalogue schema
+
+**WARNING**
+    Modifying the Catalogue schema means that a new version of the schema has to be created.
+
+**DANGER**
+    Do not modify any .sql files without changing the version of the schema first (follow the following step 1) ! If you do, the current schema will be overwritten by the modifications you made. (Rollbackable by removing your changes and by building CTA again). If any doubts, do a git diff and check that the current schema directory located in the catalogue folder does not contain any changes.
+
+In order to modify the Catalogue schema, please follow the following steps.
+
+1. Modify the **CTA_CATALOGUE_SCHEMA_VERSION_MAJOR** and the **CTA_CATALOGUE_SCHEMA_VERSION_MINOR** variables that are located in the directory *cmake/CTAVersions.cmake*.
+2. Run the build of CTA
+
+    --> It will create a new folder that will be named according to the schema versions variables modified at step 1. (Example: if the new schema version is 1.1, the directory 1.1 will be created in the catalogue directory).
+
+3. Modify the schema by editing the .sql files located in the catalogue directory :
+    - *databasetype*_catalogue_schema_header.sql
+    - common_catalogue_schema.sql
+    - *databasetype*_catalogue_schema_trailer.sql
+
+    Where *databasetype* is either **oracle** or **postgres** or **sqlite** or **mysql**.
+
+4. Run the build of CTA
+    --> It will modify the files located in the folder created at step 2.
+
+5. Try the new schema you created in an empty database. Your schema should work with Oracle, PostgreSQL, MySQL and SQLite.
+
+**TIP**
+        You can use the **cta-catalogue-schema-create** tool to create the new schema
+
+6. If everything works, let's define a [upgrade strategy](https://eoscta.docs.cern.ch/catalogue/upgrade/).
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