docs: primary-key

Signed-off-by: Raju Udava <86527202+dstala@users.noreply.github.com>

packages/noco-docs/content/en/setup-and-usages/primary-key.md

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+---
+title: "Primary Key"
+description: "Primary Key"
+position: 574
+category: "Product"
+menuTitle: "Primary Key"
+---
+
+## What is a Primary Key ?
+- Key in a relational database table that is unique for each record
+
+## What is the use of Primary Key ?
+- As it uniquely identifies an individual record of a table, it is used internally by NocoDB for all operations associated with a record
+
+## Primary Key in NocoDB
+- Primary Key that gets defined / used in NocoDB depends on how underlying table was created. Summary is captured below
+1. From UI, Create new table / Import from Excel / Import from CSV
+    1. An `ID` [datatype: Integer] system field created by default during table creation is used as primary key
+    2. Additional system fields `created-at`, `updated-at` are inserted by default & can be omitted optionally; these fields can be deleted after table creation
+2. Connect to existing external database
+    1. Existing `primary key` field defined for a table is retained as is; NocoDB doesn't insert a new ID field
+    2. Additional system fields `created-at`, `updated-at` are not inserted by default
+3. Import from Airtable
+    1. Airtable record ID is marked as primary key for imported records, and is mapped to field `ncRecordId`  [datatype: varchar]
+    2. If a new record is inserted after migration & if ncRecordId field was omitted during record insertion - auto generated string will be inserted by NocoDB
+    3. Computed hash value for the entire record is stored in system field `ncRecordHash`
+    4. Additional system fields `created-at`, `updated-at` are not inserted by default
+4. Create new table using SDK / API
+    1. No default primary key field is introduced by NocoDB. It has to be explicitly specified during table creation (using attribute `pk: true`)
+
+## What if Primary Key was missing?
+It is possible to have a table without any primary key. 
+- External database table can be created without primary key configuration.
+- New table can be created using SDK / API without primary key
+In such scenario's, new records can be created in NocoDB for this table, but records can't be updated or deleted [as there is now way for NocoDB to uniquely identify these records]
+
+#### Example : Primary Key & optional system fields during new table creation
+
+
+#### Example : Show System Fields
+
+
+## Can I change the Primary Key to another column within tables ?
+- You can't update Primary Key from NocoDB UI. You can reconfigure it at database level directly & trigger `metasync` explicitly