sql - 将 postgres 字符串列转换为 json

Question

我有一个用户表，我想在其中使用 {"oldfield": < 当前行中 foo 的值 >} 将字符串列 foo 转换为 json，但我似乎找不到正确的语法。我直接使用 psql 命令行界面。

这是我尝试过的:

 ALTER TABLE user_schema.user ALTER COLUMN "foo" SET DATA TYPE json USING '{"oldfield": "foo"}';

这会将字段更改为准确的 {"oldfield":"foo"}，而不是 foo 列的实际值。我还尝试了以下组合，但都因语法错误而失败。有任何想法吗？谢谢

   ALTER TABLE user_schema.user ALTER COLUMN "foo" SET DATA TYPE json USING '{"oldfield": foo}';
ERROR:  invalid input syntax for type json
DETAIL:  Token "foo" is invalid.
CONTEXT:  JSON data, line 1: {"oldfield": foo...

 ALTER TABLE user_schema.user ALTER COLUMN "foo" SET DATA TYPE json USING '{"oldfield": \"foo\"}';
ERROR:  invalid input syntax for type json
DETAIL:  Token "\" is invalid.
CONTEXT:  JSON data, line 1: {"oldfield": \...
ALTER TABLE user_schema.user ALTER COLUMN "foo" SET DATA TYPE json USING '{"oldfield": 'foo'}';
ERROR:  syntax error at or near "foo"
LINE 1: ...Metadata" SET DATA TYPE json USING '{"oldfield": 'foo...
                                                             ^
ALTER TABLE user_schema.user ALTER COLUMN "foo" SET DATA TYPE json USING '{"oldfield": `foo`}';
ERROR:  invalid input syntax for type json
DETAIL:  Token "`" is invalid.
CONTEXT:  JSON data, line 1: {"oldfield": `...
 ALTER TABLE user_schema.user ALTER COLUMN "foo" SET DATA TYPE json USING '{"oldfield": user.foo}';
ERROR:  invalid input syntax for type json
DETAIL:  Token "user" is invalid.
CONTEXT:  JSON data, line 1: {"oldfield": user...
ALTER TABLE user_schema.user ALTER COLUMN "foo" SET DATA TYPE json USING '{"oldfield": user_schema.user.foo}';
ERROR:  invalid input syntax for type json
DETAIL:  Token "user_schema" is invalid.
CONTEXT:  JSON data, line 1: {"oldfield": user_schema...
ALTER TABLE user_schema.user ALTER COLUMN "foo" SET DATA TYPE json USING '{"oldfield": foo}';
ERROR:  invalid input syntax for type json
DETAIL:  Token "foo" is invalid.

Answer 1

使用函数 json_build_object():

create table my_table(foo text);
insert into my_table values
    ('first'),
    ('second');

alter table my_table 
    alter column foo set data type json 
    using json_build_object('oldfield', foo);

select *
from my_table;

           foo           
-------------------------
 {"oldfield" : "first"}
 {"oldfield" : "second"}
(2 rows)