> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/15/the-sql-language/type-conversion/10.6.-select-shu-chu-gui-ze.md).

# 10.6. SELECT 輸出規則

The rules given in the preceding sections will result in assignment of non-`unknown`data types to all expressions in a SQL query, except for unspecified-type literals that appear as simple output columns of a`SELECT`command. For example, in

```
SELECT 'Hello World';
```

there is nothing to identify what type the string literal should be taken as. In this situationPostgreSQLwill fall back to resolving the literal's type as`text`.

When the`SELECT`is one arm of a`UNION`(or`INTERSECT`or`EXCEPT`) construct, or when it appears within`INSERT ... SELECT`, this rule is not applied since rules given in preceding sections take precedence. The type of an unspecified-type literal can be taken from the other`UNION`arm in the first case, or from the destination column in the second case.

`RETURNING`lists are treated the same as`SELECT`output lists for this purpose.

## Note

Prior toPostgreSQL10, this rule did not exist, and unspecified-type literals in a`SELECT`output list were left as type`unknown`. That had assorted bad consequences, so it's been changed.


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