Check out example codes for "alter domain sql". It will help you in understanding the concepts better.

Code Example 1

ALTER DOMAIN { ime | staro_ime TO novo_ime }{
[SET DEFAULT {literal | NULL | USER | ….}]
| [DROP DEFAULT]
| [ADD [CONSTRAINT] CHECK (<pogoji>)]
| [DROP CONSTRAINT]
|TYPE tip_domene };


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Primer: 
ALTER DOMAIN DTip_davka TO Domena_DDV;
ALTER DOMAIN Dspol SET DEFAULT 'M';

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