PostgreSQL中crosstab实现动态行转列（附案例）
行转列，通用做法是使用 case when 。today，介绍一下新的方法 -- crosstab
crosstab 函数被用来生成 pivot（扭曲，转动）展示，即通过横向而不是下拉展示（行转列）。如下面这个例子：
源表：
row1 val11
row1 val12
row1 val13
...
row2 val21
row2 val22
row2 val23
...
期望的结果：
row1 val11 val12 val13 ...
row2 val21 val22 val23 ...
...
扩展tablefunc
PostgreSQL 提供了一个 tablefunc 模块，内置了多个函数，其中就有 crosstab（交叉表，又叫行转列，或者长表转宽表）
--prerequisite: 安装插件tablefunc
create extension tablefunc;
crosstab声明该函数返回setof record，因此必须在FROM在调用SELECT语句的子句中定义输出列的实际名称和类型，例如：
SELECT * FROM crosstab('...') AS ct(row_name text, _col1 text, _col2 text);产生一个类似于的集合:
row_name _col1 _col2
----------+------------+------------
row1 val1 val2
row2 val3 val4案例：
需求：在动态行转列不清楚生成的宽表有多少列的情况下，或者列太多，手动敲很不友好，那么可以借助创建函数（pivotcode）来简化工作：
创建函数pivotcode：
---pivotcode (tablename varchar, rowc varchar, colc varchar, cellc varchar, celldatatype varchar)
---pivotcode(tablename：要透视的源表的名称,rowc：要成为行的源表中的列的名称,colc：想成为列的源表中的列的名称,cellc：确定将如何创建单元格值的聚合表达式,celldatatype：单元格所需的数据类型)
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
create or replace function pivotcode (tablename varchar, rowc varchar, colc varchar, cellc varchar, celldatatype varchar) returns varchar language plpgsql as $$
declare
dynsql1 varchar;
dynsql2 varchar;
columnlist varchar;
begin
-- 1. retrieve list of column names.
dynsql1 = 'select string_agg(distinct ''_''||'||colc||'||'' '||celldatatype||''','','' order by ''_''||'||colc||'||'' '||celldatatype||''') from '||tablename||';';
execute dynsql1 into columnlist;
-- 2. set up the crosstab query
dynsql2 = 'select * from crosstab (
''select '||rowc||','||colc||','||cellc||' from '||tablename||' group by 1,2 order by 1,2'',
''select distinct '||colc||' from '||tablename||' order by 1''
)
as newtable (
'||rowc||' varchar,'||columnlist||'
);';
return dynsql2;
end
$$创建表并插入数据：
create table table_to_pivot (
rowname varchar,
colname varchar,
cellval numeric
);
insert into table_to_pivot values ('row1','col1',11);
insert into table_to_pivot values ('row1','col2',12);
insert into table_to_pivot values ('row1','col3',13);
insert into table_to_pivot values ('row2','col1',21);
insert into table_to_pivot values ('row2','col2',22);
insert into table_to_pivot values ('row2','col3',23);
insert into table_to_pivot values ('row3','col1',31);
insert into table_to_pivot values ('row3','col2',32);
insert into table_to_pivot values ('row3','col3',33);
insert into table_to_pivot values ('row1','col1',141);
insert into table_to_pivot values ('row1','col2',122);
insert into table_to_pivot values ('row4','col3',131);
insert into table_to_pivot values ('row1','col1',211);
insert into table_to_pivot values ('row3','col2',222);
insert into table_to_pivot values ('row5','col3',163);
insert into table_to_pivot values ('row7','col8',316);
insert into table_to_pivot values ('row5','col2',324);
insert into table_to_pivot values ('row33','col3',233);
使用函数pivotcode：
select pivotcode('table_to_pivot','rowname','colname','max(cellval)','integer')
执行 pivotcode 生成的 SQL ：
select * from crosstab (
'select rowname,colname,max(cellval) from table_to_pivot group by 1,2 order by 1,2',
'select distinct colname from table_to_pivot order by 1'
)
as newtable (
rowname varchar,_col1 integer,_col2 integer,_col3 integer,_col8 integer
);运行SQL得到透视表：
函数pivotcode，有以下缺点：
-
如果返回的列有空格，需要自己手工修改
-
可能会有大量的 Null 值，而不是 0
-
返回的是一个 SQL，您需要复制出来后自己再执行
update版（实现pivotcode函数返回 json数据）：
直接通过pivotcode函数返回结果，而不是返回一个待执行的 SQL 语句
-- PL/pgSQL code to create pivot tables with automatic column names
-- prerequisite: install the tablefunc module
create extension if not exists tablefunc;
-- tablename: name of source table you want to pivot
-- rowc: the name of the column in source table you want to be the rows
-- colc: the name of the column in source table you want to be the columns
-- cellc: an aggregate expression determining how the cell values will be created
-- celldatatype: desired data type for the cells
create or replace function pivot (tablename varchar, rowc varchar, colc varchar, cellc varchar, celldatatype varchar)
returns json language plpgsql as $$
declare
dynsql1 varchar;
dynsql2 varchar;
columnlist varchar;
stmt TEXT;
result json;
begin
-- 1. retrieve list of column names.
dynsql1 = 'select string_agg(distinct ''_''||'||colc||'||'' '||celldatatype||''','','' order by ''_''||'||colc||'||'' '||celldatatype||''') from '||tablename||';';
execute dynsql1 into columnlist;
-- 2. set up the crosstab query
dynsql2 = 'select * from crosstab (
''select '||rowc||','||colc||','||cellc||' from '||tablename||' group by 1,2 order by 1,2'',
''select distinct '||colc||' from '||tablename||' order by 1''
)
as newtable (
'||rowc||' varchar,'||columnlist||'
)';
stmt = E' select array_to_json(array_agg(row_to_json(t))) from ('|| dynsql2 ||') t ';
execute stmt into result;
return result;
end
$$select pivot('table_to_pivot','rowname','colname','max(cellval)','integer')
json结果数据：
[
{"rowname":"row1","_col1":211,"_col2":122,"_col3":13,"_col8":null},
{"rowname":"row2","_col1":21,"_col2":22,"_col3":23,"_col8":null},
{"rowname":"row3","_col1":31,"_col2":222,"_col3":33,"_col8":null},
{"rowname":"row33","_col1":null,"_col2":null,"_col3":233,"_col8":null},
{"rowname":"row4","_col1":null,"_col2":null,"_col3":131,"_col8":null},
{"rowname":"row5","_col1":null,"_col2":324,"_col3":163,"_col8":null},
{"rowname":"row7","_col1":null,"_col2":null,"_col3":null,"_col8":316}
]参考文献 ：
postgresql11：https://www.postgresql.org/docs/11/tablefunc.html
Pivot函数: https://madlib.apache.org/docs/latest/group__grp__pivot.html
pivotmytable: https://github.com/jtornero/pivotmytable
crosstabview: https://wiki.postgresql.org/wiki/Crosstabview
dynamic_pivot: https://postgresql.verite.pro/blog/2018/06/19/crosstab-pivot.html