sql - Postgres 动态列标题(来自另一个表)

Question

只是一个愚蠢的例子:

Table A:

1. eggs
2. bread
3. cheese

Table B (when they are eaten):

1. Egg | date
2. Bread | date
3. Egg | date
4. cheese | date
5. Bread | date

出于统计目的，我需要按日期对每种食物类型进行统计，如下所示:

表统计:

             egg   |   bread   |   cheese

date1         2          1            0    

date2         6          4            2

date3         2          0            0

我需要报告中的列标题是动态的(如果添加了新的，它应该会自动出现)。

知道如何在 postgres 中实现这个吗？

谢谢。

Answer 1

基于答案Postgres dynamic column headers (from another table) (Eric Vallabh Minikel 的作品)我改进了功能，使其更加灵活方便。我认为它也可能对其他人有用，特别是因为它只依赖于 pg/plsql 并且不需要像 erics 的其他派生工作(即 plpython)那样安装扩展。使用 9.3.5 进行测试，但至少也应该可以工作到 9.2。

改进:

• 处理包含空格的旋转列名
• 处理多行标题列
• 处理数据透视单元格和非聚合数据透视单元格中的聚合函数(最后一个参数可能是“sum(cellval)”以及“cellval”，以防基础表/ View 已经进行聚合)
• 自动检测主元格的数据类型(不再需要传递给函数)

用法:

SELECT get_crosstab_statement('table_to_pivot', ARRAY['rowname' [, <other_row_header_columns_as_well>], 'colname', 'max(cellval)');

代码:

CREATE OR REPLACE FUNCTION get_crosstab_statement(tablename character varying, row_header_columns character varying[], pivot_headers_column character varying, pivot_values character varying)
  RETURNS character varying AS
$BODY$
--returns the sql statement to use for pivoting the table
--based on: http://www.cureffi.org/2013/03/19/automatically-creating-pivot-table-column-names-in-postgresql/
--based on: https://stackoverflow.com/questions/4104508/postgres-dynamic-column-headers-from-another-table
--based on: http://www.postgresonline.com/journal/categories/24-tablefunc

DECLARE
    arrayname CONSTANT character varying := 'r';

    row_headers_simple character varying;
    row_headers_quoted character varying;
    row_headers_castdown character varying;
    row_headers_castup character varying;
    row_header_count smallint;
    row_header record;

    pivot_values_columnname character varying;
    pivot_values_datatype character varying;
    pivot_headers_definition character varying;
    pivot_headers_simple character varying;

    sql_row_headers character varying;
    sql_pivot_headers character varying;
    sql_crosstab_result character varying;

BEGIN
    -- 1. create row header definitions
    row_headers_simple   :=         array_to_string(row_header_columns, ', ');
    row_headers_quoted   := '''' || array_to_string(row_header_columns, ''', ''') || '''';
    row_headers_castdown :=         array_to_string(row_header_columns, '::text, ') || '::text';

    row_header_count     := 0;
    sql_row_headers      := 'SELECT column_name, data_type
                            FROM information_schema.columns
                            WHERE table_name = ''' || tablename || ''' AND column_name IN (' || row_headers_quoted || ')';
    FOR row_header IN EXECUTE sql_row_headers LOOP
        row_header_count := row_header_count + 1;
        row_headers_castup := COALESCE(row_headers_castup || ', ', '') || arrayname || '[' || row_header_count || ']::' || row_header.data_type || ' AS ' || row_header.column_name;
    END LOOP;

    -- 2. retrieve basic column name in case an aggregate function is used
    SELECT coalesce(substring(pivot_values FROM '.*\((.*)\)'), pivot_values)
    INTO pivot_values_columnname;

    -- 3. retrieve pivot values datatype
    SELECT data_type
    FROM information_schema.columns
    WHERE table_name = tablename AND column_name = pivot_values_columnname
    INTO pivot_values_datatype;

    -- 4. retrieve list of pivot column names.
    sql_pivot_headers := 'SELECT string_agg(DISTINCT quote_ident(' || pivot_headers_column || '), '', '' ORDER BY quote_ident(' || pivot_headers_column || ')) as names, string_agg(DISTINCT quote_ident(' || pivot_headers_column || ') || '' ' || pivot_values_datatype || ''', '', '' ORDER BY quote_ident(' || pivot_headers_column || ') || '' ' || pivot_values_datatype || ''') as definitions FROM ' || tablename || ';';
    EXECUTE sql_pivot_headers INTO pivot_headers_simple, pivot_headers_definition;

    -- 5. set up the crosstab query
    sql_crosstab_result := 'SELECT  ' || replace (row_headers_castup || ', ' || pivot_headers_simple, ', ', ',
        ') || '
FROM    crosstab (
        ''SELECT ARRAY[' || row_headers_castdown || '] AS ' || arrayname || ', ' || pivot_headers_column || ', ' || pivot_values || '
        FROM ' || tablename || '
        GROUP BY ' || row_headers_simple || ', ' || pivot_headers_column || (CASE pivot_values_columnname=pivot_values WHEN true THEN ', ' || pivot_values ELSE '' END) || '
        ORDER BY ' || row_headers_simple || '''
    ,
        ''SELECT DISTINCT ' || pivot_headers_column || '
        FROM ' || tablename || '
        ORDER BY ' || pivot_headers_column || '''
    ) AS newtable (
        ' || arrayname || ' varchar[]' || ',
        ' || replace(pivot_headers_definition, ', ', ',
        ') || '
    );';

    RETURN sql_crosstab_result;
 END

 $BODY$
  LANGUAGE plpgsql STABLE
  COST 100;