Refactor CaseExpression

[Tigrov]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️
Fixed issues

Related PRs

• Adapt test for changes in db db-sqlite#392
• Adapt test for changes in db db-mysql#429
• Adapt for changes in db db-pgsql#448
• Adapt test for changes in db db-mssql#399
• Adapt test for changes in db db-oracle#366

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.40%. Comparing base (f8c61a9) to head (c2f167e).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1053      +/-   ##
============================================
- Coverage     98.41%   98.40%   -0.01%     
+ Complexity     1647     1641       -6     
============================================
  Files           115      115              
  Lines          4355     4337      -18     
============================================
- Hits           4286     4268      -18     
  Misses           69       69              

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