Create a replacement values table, and use it to do the data replacement.

CREATE TABLE dbo.data_replacements (

column_name varchar(30) NOT NULL,

current_value varchar(100) NULL,

replacement_value varchar(100) NULL,

CONSTRAINT data_replacements__CL UNIQUE CLUSTERED ( column_name, current_value )

)

INSERT INTO dbo.data_replacements ( column_name, current_value, replacement_value )

SELECT 'city', 'JRK_Ikosium', 'Icosium' UNION ALL

SELECT 'city', 'JRK_Géryville', 'El_Bayadh' UNION ALL

SELECT 'city', 'JRK_Cirta', 'Constantine' UNION ALL

SELECT 'city', 'JRK_Rusicade', 'Philippeville' UNION ALL

SELECT 'city', 'JRK_Saldae', 'Bougie'

select t.id, t.city, COALESCE(city_replacement.replacement_value, t.city) AS New_city_name

from towns t

outer apply (

select top (1) dr.replacement_value

from dbo.data_replacements dr

where

dr.column_name = 'city' and

dr.current_value = t.city

) as city_replacement