Database way to how find all duplicate custom/system fields in JIRA

[Dayanand]

1. May I know Database procedure to merge and remove custom fields without any side impact.

2 May I know Database procedure to merge and remove system fields without any side impact.

i.e how we handle removing both custom and system fields in faster manner since UI will take more effort. we have 200 duplicate custom fields.

[Nic Brough -Adaptavist-]

1. There isn't a "merge" or "remove" procedure in the database.  You will need to run a large project to work out how to do it completely safely and without side effects, which is likely to be weeks or months of work, during which time you will not be able to use Jira.
2. System fields cannot be merged or removed 

For merging custom fields, I would look to the apps in the marketplace designed for housekeeping and cleaning.   Power Admin and Optimiser are the first two I would take a look at (and note they'll help you simplify in other ways). 

If you want to do it yourself, then use a combination of the easier more simple UI bits and for the more onerous parts (mostly the merges), script it with the REST API - no downtime, no risk (beyond human error of doing the wrong thing), and very little coding needed.

[Dayanand]

@Nic Brough -Adaptavist- 

May i know the REST API way to do this activity.