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 ...
现在我想要如下输出:
Sl.no 1 2 3 4 ...
Username A B C D ...
Designation XYZ RTS QWE HGD ...
Salary 10000 50000 20000 34343 ...
如何做到这一点?
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:将列转换为行
我的答案基于以下表格:
CREATE TABLE tbl (
sl_no int
, username text
, designation text
, salary int
);
每行都会返回一个新列。对于这样的动态返回类型,几乎不可能通过对数据库的单次调用使其完全动态化。通过两个步骤演示解决方案:
通常,这受到表可以容纳的最大列数的限制。因此,对于行数超过 1600 行(或更少)的表来说,这不是一个选择。详情:
crosstab()
如果可以的话,使用第一个。击败其他人。
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
而不是实际的列名称进行操作。更简单、更快。再次将结果连接到 pg_attribute
或像第 9.3 页示例中那样集成列名称。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。太多了,无法解释。
unnest()
unnest()
现在可以并行取消嵌套多个数组。
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);
db<>小提琴这里
老sqlfiddle
crosstab()
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;
可以包装到具有单个参数的函数中...
生成以下形式的查询:
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);
产生期望的结果:
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
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;
生成以下形式的查询:
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
同样的结果。
如果(像我一样)您需要来自 bash 脚本的此信息,请注意 psql 有一个简单的命令行开关来告诉它将表列输出为行:
psql mydbname -x -A -F= -c "SELECT * FROM foo WHERE id=123"
-x
选项是让psql将列输出为行的关键。
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,这取决于您要应用此的情况。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。即:
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)
这是一种更易于维护的方法,恕我直言。查看此页面了解更多信息:https://www.postgresql.org/docs/current/typeconv-union-case.html
在普通 SQL 或 PL/pgSQL 中没有正确的方法来执行此操作。
在从数据库获取数据的应用程序中执行此操作会更好。