Question

我需要从两个或更多表中选择行（“A”，“B”）。他们有差异列，我不使用继承。

因此。例如：

SELECT * FROM "A" UNION SELECT * FROM "B"

ERROR: each UNION query must have the same number of columns

我能理解为什么。

我尝试从根表中的根模式获取相交的列：

SELECT column_name FROM information_schema.columns
WHERE table_schema = 'client_root' AND table_name ='conditions'

没关系！但我不使用查询：

SELECT
   (SELECT column_name FROM information_schema.columns
    WHERE table_schema = 'client_root' AND table_name ='conditions')
FROM "client_123"."A"

因此。如何将子选择数据放在root选择中？

Answer 1

你想做的事情完全不可能。

创建动态SQL

首先，这是你可以做的事情：一个为这样的查询创建SQL的plpgsql函数：

CREATE OR REPLACE FUNCTION f_union_common_col_sql(text, text)
 RETURNS text
AS $function$
DECLARE 
  _cols text;
BEGIN

_cols := string_agg(attname, ', ')
FROM (
    SELECT a.attname
    FROM   pg_attribute a
    WHERE  a.attrelid = $1::regclass::oid
    AND    a.attnum >= 1
    INTERSECT
    SELECT a.attname
    FROM   pg_attribute a
    WHERE  a.attrelid = $2::regclass::oid
    AND    a.attnum >= 1
    ) x;

RETURN 'SELECT ' || _cols || '
FROM   ' || quote_ident($1) || '
UNION
SELECT ' || _cols || '
FROM   ' || quote_ident($1);

END;
$function$  LANGUAGE plpgsql;

COMMENT ON FUNCTION f_union_common_col_sql(text, text) IS 'Create SQL to query all visible columns that two tables have in common.
# Without duplicates. Use UNION ALL if you want to include duplicates.
# Depends on visibility dicatated by search_path
$1 .. table1: optionally schema-qualified, case sensitive!
$2 .. table2: optionally schema-qualified, case sensitive!';

致电：

SELECT f_union_common_col_sql('myschema1.tbl1', 'myschema2.tbl2');

为您提供完整的查询。在第二次通话中执行。

你可以在manual on plpgsql functions找到我在这里使用的大部分内容 PostgreSQL 9.0引入了aggregate function string_agg()。在旧版本中，您可以：array_to_string(array_agg(attname), ', ')。

执行动态SQL？

接下来，这是几乎无法做的事情：

CREATE OR REPLACE FUNCTION f_union_common_col(text, text)
  RETURNS SETOF record AS
$BODY$
DECLARE 
  _cols text;
BEGIN

_cols := string_agg(attname, ', ')
FROM (
    SELECT a.attname
    FROM   pg_attribute a
    WHERE  a.attrelid = $1::regclass::oid
    AND    a.attnum >= 1
    INTERSECT
    SELECT a.attname
    FROM   pg_attribute a
    WHERE  a.attrelid = $2::regclass::oid
    AND    a.attnum >= 1
    ) x;

RETURN QUERY EXECUTE '
SELECT ' || _cols || '
FROM quote_ident($1)
UNION
SELECT ' || _cols || '
FROM quote_ident($2)';

END;
$BODY$
  LANGUAGE plpgsql VOLATILE;

COMMENT ON FUNCTION f_union_common_col(text, text) IS 'Query all visible columns that two tables have in common.
# Without duplicates. Use UNION ALL if you want to include duplicates.
# Depends on visibility dicatated by search_path
# !BUT! you need to specify a column definition list for every call. So, hardly useful.
$1 .. table1 (optionally schema-qualified)
$2 .. table1 (optionally schema-qualified)';

函数调用要求您指定目标列的列表。所以这根本没什么用处：

SELECT * from f_union_common_col('myschema1.tbl1', 'myschema2.tbl2')

ERROR:  a column definition list is required for functions returning "record"

没有简单的方法可以解决这个问题。您必须动态创建函数或至少复杂类型。这是我停下的地方。

在PostgreSQL中没有函数的SELECT动态列

1 个答案:

创建动态SQL

执行动态SQL？