PostgreSQL将列转换为行？移调？

Question

我有一个PostgreSQL函数(或表),它给我以下输出:

Sl.no    username    Designation    salary   etc..
 1        A           XYZ            10000    ...
 2        B           RTS            50000    ...
 3        C           QWE            20000    ...
 4        D           HGD            34343    ...

现在我想要输出如下:

Sl.no            1       2        3       4       ...
 Username        A       B        C       D       ...
 Designation     XYZ     RTS      QWE     HGD     ...
 Salary          10000   50000    20000   34343   ...

这该怎么做？

Answer 1

将我的答案基于表格的表格:

CREATE TABLE tbl (
   sl_no int
 , username text
 , designation text
 , salary int
);

每行都会返回一个新列.使用这样的动态返回类型,通过单次调用数据库几乎不可能使其完全动态化.通过两个步骤演示解决方案:

1. 生成查询
2. 执行生成的查询

通常,这受到表可以容纳的最大列数的限制.因此,对于超过1600行(或更少)的表,这不是一个选项.细节:

• PostgreSQL选择查询中的最大列数是多少

Postgres 9.3或更早

动态解决方案 crosstab()

• 完全动态,适用于任何桌子.在两个地方提供表名:

SELECT 'SELECT *
FROM   crosstab(
       ''SELECT unnest(''' || quote_literal(array_agg(attname))
                           || '''::text[]) AS col
             , row_number() OVER ()
             , unnest(ARRAY[' || string_agg(quote_ident(attname)
                              || '::text', ',') || ']) AS val
        FROM   ' || attrelid::regclass || '
        ORDER  BY generate_series(1,' || count(*) || '), 2''
   ) t (col text, '
     || (SELECT string_agg('r'|| rn ||' text', ',')
         FROM (SELECT row_number() OVER () AS rn FROM tbl) t)
     || ')' AS sql
FROM   pg_attribute
WHERE  attrelid = 'tbl'::regclass
AND    attnum > 0
AND    NOT attisdropped
GROUP  BY attrelid;

可以用单个参数包装到函数中...
生成表单的查询:

SELECT *
FROM   crosstab(
       'SELECT unnest(''{sl_no,username,designation,salary}''::text[]) AS col
             , row_number() OVER ()
             , unnest(ARRAY[sl_no::text,username::text,designation::text,salary::text]) AS val
        FROM   tbl
        ORDER  BY generate_series(1,4), 2'
   ) t (col text, r1 text,r2 text,r3 text,r4 text)

产生预期的结果:

col         r1    r2      r3     r4
-----------------------------------
sl_no       1      2      3      4
username    A      B      C      D
designation XYZ    RTS    QWE    HGD
salary      10000  50000  20000  34343

简单的解决方案 unnest()

SELECT 'SELECT unnest(''{sl_no, username, designation, salary}''::text[] AS col)
     , ' || string_agg('unnest('
                    || quote_literal(ARRAY[sl_no::text, username::text, designation::text, salary::text])
                    || '::text[]) AS row' || sl_no, E'\n     , ') AS sql
FROM   tbl;

• 对于包含多列的表格来说速度很慢.

生成表单的查询:

SELECT unnest('{sl_no, username, designation, salary}'::text[]) AS col
     , unnest('{10,Joe,Music,1234}'::text[]) AS row1
     , unnest('{11,Bob,Movie,2345}'::text[]) AS row2
     , unnest('{12,Dave,Theatre,2356}'::text[]) AS row3
     , unnest('{4,D,HGD,34343}'::text[]) AS row4

结果相同.

Postgres 9.4+

动态解决方案 crosstab()

如果可以,请使用此选项.打败其余的.

SELECT 'SELECT *
FROM   crosstab(
       $ct$SELECT u.attnum, t.rn, u.val
        FROM  (SELECT row_number() OVER () AS rn, * FROM '
                              || attrelid::regclass || ') t
             , unnest(ARRAY[' || string_agg(quote_ident(attname)
                              || '::text', ',') || '])
                 WITH ORDINALITY u(val, attnum)
        ORDER  BY 1, 2$ct$
   ) t (attnum bigint, '
     || (SELECT string_agg('r'|| rn ||' text', ', ')
         FROM  (SELECT row_number() OVER () AS rn FROM tbl) t)
     || ')' AS sql
FROM   pg_attribute
WHERE  attrelid = 'tbl'::regclass
AND    attnum > 0
AND    NOT attisdropped
GROUP  BY attrelid;

使用attnum而不是实际的列名来操作.更简单,更快捷.将结果pg_attribute再次加入或集成列名,如第9.3页示例所示.
生成表单的查询:

SELECT *
FROM   crosstab(
       $ct$SELECT u.attnum, t.rn, u.val
        FROM  (SELECT row_number() OVER () AS rn, * FROM tbl) t
             , unnest(ARRAY[sl_no::text,username::text,designation::text,salary::text])
                WITH ORDINALITY u(val, attnum)
        ORDER  BY 1, 2$ct$
   ) t (attnum bigint, r1 text, r2 text, r3 text, r4 text);

这使用了一系列高级功能.太难解释了.

简单的解决方案 unnest()

一个unnest()现在可以利用多个阵列在平行于UNNEST.

SELECT 'SELECT * FROM unnest(
  ''{sl_no, username, designation, salary}''::text[]
, ' || string_agg(quote_literal(ARRAY[sl_no::text, username::text, designation::text, salary::text])
              || '::text[]', E'\n, ')
    || E') \n AS t(col,' || string_agg('row' || sl_no, ',') || ')' AS sql
FROM   tbl;

结果:

SELECT * FROM unnest(
 '{sl_no, username, designation, salary}'::text[]
,'{10,Joe,Music,1234}'::text[]
,'{11,Bob,Movie,2345}'::text[]
,'{12,Dave,Theatre,2356}'::text[])
 AS t(col,row1,row2,row3,row4)

SQL Fiddle在第9.3页上运行.

Answer 2

SELECT
   unnest(array['Sl.no', 'username', 'Designation','salary']) AS "Columns",
   unnest(array[Sl.no, username, value3Count,salary]) AS "Values"
FROM view_name
ORDER BY "Columns"

参考:convertingColumnsToRows

Answer 3

如果(像我一样)你需要来自bash脚本的这些信息,请注意psql有一个简单的命令行开关,告诉它将表列输出为行:

psql mydbname -x -A -F= -c "SELECT * FROM foo WHERE id=123"

该-x选项是让psql将列输出为行的关键.

Answer 4

我有一个比 Erwin 上面指出的更简单的方法，它对我来说适用于 Postgres（而且我认为它应该适用于所有支持 SQL 标准的主要关系数据库）

您可以简单地使用 UNION 而不是交叉表：

SELECT text 'a' AS "text" UNION SELECT 'b';

 text
------
 a
 b
(2 rows)

当然，这取决于您要应用它的情况。考虑到您事先知道需要哪些字段，因此即使查询不同的表也可以采用这种方法。IE：

SELECT 'My first metric' as name, count(*) as total from first_table UNION
SELECT 'My second metric' as name, count(*) as total from second_table 

 name             | Total
------------------|--------
 My first metric  |     10
 My second metric |     20
(2 rows)

恕我直言，这是一种更易于维护的方法。查看此页面以获取更多信息：https://www.postgresql.org/docs/current/typeconv-union-case.html