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          alter table xxx rename column aaa to bbb;
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          alter table xxx drop column aaa;
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          alter table xxx add aaa varchar2(22);
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           alter table xxx modify aaa varchar2(22);
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           update xxx set aaa = bbb;
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           alter table xxx modify aaa (not) null;
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           alter table xxx modify aaa default 1;
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           comment on column xxx.aaa  is 'abc......';
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           alter table xxx add constraint PK_XXX primary key (aaa);
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           alter table xxx drop constraint PK_XXX cascade;
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           create (unique) index IDX_XXX_AAA on xxx(aaa);
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