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Otherwise it behaves like the underlying type — for example, any operator or function that can be applied to the underlying type will work on the domain type. The underlying type can be any built-in or user-defined base type, enum type, array type, composite type, range type, or another domain.\u003c/p\u003e\n\u003cp\u003eFor example, we could create a domain over integers that accepts only positive integers:\u003c/p\u003e\n\u003cpre class=\"programlisting\"\u003eCREATE DOMAIN posint AS integer CHECK (VALUE \u0026gt; 0);\nCREATE TABLE mytable (id posint);\nINSERT INTO mytable VALUES(1);   -- works\nINSERT INTO mytable VALUES(-1);  -- fails\n\u003c/pre\u003e\n\u003cp\u003eWhen an operator or function of the underlying type is applied to a domain value, the domain is automatically down-cast to the underlying type. Thus, for example, the result of \u003ccode class=\"literal\"\u003emytable.id - 1\u003c/code\u003e is considered to be of type \u003ccode class=\"type\"\u003einteger\u003c/code\u003e not \u003ccode class=\"type\"\u003eposint\u003c/code\u003e. We could write \u003ccode class=\"literal\"\u003e(mytable.id - 1)::posint\u003c/code\u003e to cast the result back to \u003ccode class=\"type\"\u003eposint\u003c/code\u003e, causing the domain's constraints to be rechecked. In this case, that would result in an error if the expression had been applied to an \u003ccode class=\"structfield\"\u003eid\u003c/code\u003e value of 1. Assigning a value of the underlying type to a field or variable of the domain type is allowed without writing an explicit cast, but the domain's constraints will be checked.\u003c/p\u003e\n\u003cp\u003eFor additional information see \u003ca class=\"xref\" href=\"/docs/18/sql-createdomain.html\" title=\"CREATE DOMAIN\"\u003e\u003cspan class=\"refentrytitle\"\u003eCREATE 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18 English manual","path":"domains.html","sha256":"82d486973ccc35d14276627a67ac76b434452f23f9ed4d8aa3fbb01fdf285fc2","url":"/docs/18/domains.html"}]},"MeasuredEvidence":{}},"Text":{"Collection":"type","Key":"domains","SourceDatabase":"center","Version":"18","Locale":"en","Title":"Domains","Summary":"A domain is a user-defined data type that is based on another underlying type . Optionally, it can have constraints that restrict its valid values to a subset of what the underlying type would allow. Otherwise it behaves like the underlying type — for example, any operator or function that can be applied to the underlying type will work on the domain type. The underlying type can be any built-in or user-defined base type, enum type, array type, composite type, range type, or another domain.","BodyHTML":"\u003cdiv id=\"DOMAINS\"\u003e\n\u003cdiv\u003e\n\u003cdiv\u003e\n\u003cdiv\u003e\n\u003ch2\u003e8.18. Domain Types \u003c/h2\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eA \u003cem\u003edomain\u003c/em\u003e is a user-defined data type that is based on another \u003cem\u003eunderlying type\u003c/em\u003e. Optionally, it can have constraints that restrict its valid values to a subset of what the underlying type would allow. Otherwise it behaves like the underlying type — for example, any operator or function that can be applied to the underlying type will work on the domain type. The underlying type can be any built-in or user-defined base type, enum type, array type, composite type, range type, or another domain.\u003c/p\u003e\n\u003cp\u003eFor example, we could create a domain over integers that accepts only positive integers:\u003c/p\u003e\n\u003cpre\u003eCREATE DOMAIN posint AS integer CHECK (VALUE \u0026gt; 0);\nCREATE TABLE mytable (id posint);\nINSERT INTO mytable VALUES(1);   -- works\nINSERT INTO mytable VALUES(-1);  -- fails\n\u003c/pre\u003e\n\u003cp\u003eWhen an operator or function of the underlying type is applied to a domain value, the domain is automatically down-cast to the underlying type. Thus, for example, the result of \u003ccode\u003emytable.id - 1\u003c/code\u003e is considered to be of type \u003ccode\u003einteger\u003c/code\u003e not \u003ccode\u003eposint\u003c/code\u003e. We could write \u003ccode\u003e(mytable.id - 1)::posint\u003c/code\u003e to cast the result back to \u003ccode\u003eposint\u003c/code\u003e, causing the domain\u0026#39;s constraints to be rechecked. In this case, that would result in an error if the expression had been applied to an \u003ccode\u003eid\u003c/code\u003e value of 1. Assigning a value of the underlying type to a field or variable of the domain type is allowed without writing an explicit cast, but the domain\u0026#39;s constraints will be checked.\u003c/p\u003e\n\u003cp\u003eFor additional information see \u003ca href=\"/docs/18/sql-createdomain.html\" title=\"CREATE DOMAIN\" rel=\"nofollow\"\u003e\u003cspan\u003eCREATE DOMAIN\u003c/span\u003e\u003c/a\u003e.\u003c/p\u003e\n\u003c/div\u003e","SourceRevision":"555610c24d53e4316da5b7d3fc25c279d96856d5e0e23ee308c328c5fa881d9f","ContentHash":"f04592c5ceb7b1d43c0d064335f4f4ac6e80745d6a5a72080f2046d12cd83632","Payload":{"description":["A domain is a user-defined data type that is based on another underlying type . Optionally, it can have constraints that restrict its valid values to a subset of what the underlying type would allow. Otherwise it behaves like the underlying type — for example, any operator or function that can be applied to the underlying type will work on the domain type. The underlying type can be any built-in or user-defined base type, enum type, array type, composite type, range type, or another domain."],"manual_html":"\u003cdiv class=\"sect1\" id=\"DOMAINS\"\u003e\n\u003cdiv class=\"titlepage\"\u003e\n\u003cdiv\u003e\n\u003cdiv\u003e\n\u003ch2 class=\"title\"\u003e8.18. Domain Types \u003c/h2\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eA \u003cem class=\"firstterm\"\u003edomain\u003c/em\u003e is a user-defined data type that is based on another \u003cem class=\"firstterm\"\u003eunderlying type\u003c/em\u003e. Optionally, it can have constraints that restrict its valid values to a subset of what the underlying type would allow. Otherwise it behaves like the underlying type — for example, any operator or function that can be applied to the underlying type will work on the domain type. The underlying type can be any built-in or user-defined base type, enum type, array type, composite type, range type, or another domain.\u003c/p\u003e\n\u003cp\u003eFor example, we could create a domain over integers that accepts only positive integers:\u003c/p\u003e\n\u003cpre class=\"programlisting\"\u003eCREATE DOMAIN posint AS integer CHECK (VALUE \u0026gt; 0);\nCREATE TABLE mytable (id posint);\nINSERT INTO mytable VALUES(1);   -- works\nINSERT INTO mytable VALUES(-1);  -- fails\n\u003c/pre\u003e\n\u003cp\u003eWhen an operator or function of the underlying type is applied to a domain value, the domain is automatically down-cast to the underlying type. Thus, for example, the result of \u003ccode class=\"literal\"\u003emytable.id - 1\u003c/code\u003e is considered to be of type \u003ccode class=\"type\"\u003einteger\u003c/code\u003e not \u003ccode class=\"type\"\u003eposint\u003c/code\u003e. We could write \u003ccode class=\"literal\"\u003e(mytable.id - 1)::posint\u003c/code\u003e to cast the result back to \u003ccode class=\"type\"\u003eposint\u003c/code\u003e, causing the domain's constraints to be rechecked. In this case, that would result in an error if the expression had been applied to an \u003ccode class=\"structfield\"\u003eid\u003c/code\u003e value of 1. Assigning a value of the underlying type to a field or variable of the domain type is allowed without writing an explicit cast, but the domain's constraints will be checked.\u003c/p\u003e\n\u003cp\u003eFor additional information see \u003ca class=\"xref\" href=\"/docs/18/sql-createdomain.html\" title=\"CREATE DOMAIN\"\u003e\u003cspan class=\"refentrytitle\"\u003eCREATE DOMAIN\u003c/span\u003e\u003c/a\u003e.\u003c/p\u003e\n\u003c/div\u003e","related":[],"sections":[]}},"RequestedLocale":"zh-Hans","Fallback":true,"Versions":["10","11","12","13","14","15","16","17","18","19","20"],"Locales":["en"],"Signatures":null,"Spellings":null,"SQLState":null,"Evidence":null}