SQL：从 2 个表创建完整记录

Question

我有一个数据库结构（为了理解问题而最大限度地简化）：

Table "entry" ("id" integer primary key)
Table "fields" ("name" varchar primary key, and others)
Table "entry_fields" ("entryid" integer primary key, "name" varchar primary key, "value")

对于给定的“entry.id”，我想获取该条目的详细信息，即。在单个 SQL 查询中链接到该条目的所有“entry_fields”。

举个例子可能会更好：

“字段”：

"result"
"output"
"code"
"command"

“条目”包含：

id : 842
id : 850

“entry_fields”包含：

entryid : 842, name : "result", value : "ok"
entryid : 842, name : "output", value : "this is an output"
entryid : 842, name : "code", value : "42"
entryid : 850, name : "result", value : "ko"
entryid : 850, name : "command", value : "print ko"

想要的输出是：

| id  | command    | output               | code | result |
| 842 | NULL       | "this is an output"  | 42   | ok     |
| 850 | "print ko" | NULL                 | NULL | ko     |

目的是能够在不改变任何“条目”表结构的情况下添加“字段”

我试过类似的东西：

SELECT e.*, (SELECT name FROM fields) FROM entry AS e

但 Postgres 抱怨：

错误：用作表达式的子查询返回多行

希望有人可以帮助我！

Answer 1

按要求解决

虽然坚持这种不幸的设计，但最快的查询是使用crosstab()，由附加模块tablefunc 提供。此相关答案中有大量详细信息：

• PostgreSQL Crosstab Query

对于提出的问题：

SELECT * FROM crosstab(
      $$SELECT e.id, ef.name, ef.value
       FROM   entry             e
       LEFT   JOIN entry_fields ef
              ON ef.entryid = e.id
             AND ef.name = ANY ('{result,output,code,command}'::text[])
       ORDER  BY 1, 2$$

     ,$$SELECT unnest('{result,output,code,command}'::text[])$$
   ) AS ct (id int, result text, output text, code text, command text);

数据库设计

如果您没有大量不同的字段，将所有三个表合并为一个简单的表会更简单、更高效：

CREATE TABLE entry (
   entry_id serial PRIMARY KEY
  ,field1   text
  ,field2   text
  , ... more fields
);

没有值的字段可以是NULL。 NULL 存储非常便宜（基本上 NULL 位图中每列 1 位）：

• How much disk-space is needed to store a NULL value using postgresql DB?
• Do nullable columns occupy additional space in PostgreSQL?

即使您有数百个不同的列，并且每个条目只填充很少的列，这仍然会使用更少的磁盘空间。

您的查询变得微不足道：

SELECT entry_id, result, output, code, command
FROM   enty;

如果您的列太多¹，这不仅仅是一种错误的设计（通常，这可以折叠成更少的列），请考虑数据类型hstore 或json / jsonb（在 Postgres 9.4 中）用于 EAV 存储。

¹Per Postgres "About" page:

Maximum Columns per Table   250 - 1600 depending on column types

考虑这个相关的答案和替代方案：

• Use case for hstore against multiple columns

关于 dba.SE 上 EAV 结构的典型用例/问题的问题：

• Is there a name for this database structure?

Answer 2

动态 SQL：

CREATE TABLE fields (name varchar(100) PRIMARY KEY)
INSERT INTO FIELDS VALUES ('RESULT')
INSERT INTO FIELDS VALUES ('OUTPUT')
INSERT INTO FIELDS VALUES ('CODE')
INSERT INTO FIELDS VALUES ('COMMAND')

CREATE TABLE ENTRY_fields (ENTRYID INT, name varchar(100), VALUE VARCHAR(100) CONSTRAINT PK PRIMARY KEY(ENTRYID, name))
INSERT INTO ENTRY_fields VALUES(842, 'RESULT', 'OK')
INSERT INTO ENTRY_fields VALUES(842, 'OUTPUT', 'THIS IS AN OUTPUT')
INSERT INTO ENTRY_fields VALUES(842, 'CODE', '42')
INSERT INTO ENTRY_fields VALUES(850, 'RESULT', 'KO')
INSERT INTO ENTRY_fields VALUES(850, 'COMMAND', 'PRINT KO')

CREATE TABLE ENTRY (ID INT PRIMARY KEY)
INSERT INTO ENTRY VALUES(842)
INSERT INTO ENTRY VALUES(850)

DECLARE @COLS NVARCHAR(MAX), @SQL NVARCHAR(MAX)

select @Cols = stuff((select ', ' + quotename(dt)
from (select DISTINCT name as dt 
from fields) X
FOR XML PATH('')),1,2,'')

PRINT @COLS
SET @SQL = 'SELECT * FROM (SELECT id, f.name, value
from fields F CROSS join ENTRY LEFT JOIN entry_fields ef on ef.name =  f.name AND ID = ef.ENTRYID
) Y PIVOT (max(value) for name in ('+ @Cols +'))PVT '

--print @SQL

exec (@SQL)

如果您认为您的值将在字段表中保持不变：

SELECT * FROM (SELECT id, f.name ,value
from fields F CROSS join ENTRY LEFT JOIN entry_fields ef on ef.name =  f.name AND ID = ef.ENTRYID
) Y PIVOT (max(value) for name in ([CODE], [COMMAND], [OUTPUT], [RESULT]))PVT

可以使用 postgresql 的查询：

    SELECT ID, MAX(CODE) as CODE, MAX(COMMAND) as COMMAND, MAX(OUTPUT) as OUTPUT, MAX(RESULT) as RESULT
FROM (SELECT ID, 
      CASE WHEN f.name = 'CODE' THEN VALUE END AS CODE, 
      CASE WHEN f.name = 'COMMAND' THEN VALUE END AS COMMAND,
      CASE WHEN f.name = 'OUTPUT' THEN VALUE END AS OUTPUT,
      CASE WHEN f.name = 'RESULT' THEN VALUE END AS RESULT
from fields F CROSS join ENTRY LEFT JOIN entry_fields ef on ef.name =  f.name AND ID = ENTRYID
) Y
GROUP BY ID

Answer 3

子查询 (SELECT name FROM fields) 将返回 4 行。您不能在 SQL 中将 4 行填充到 1 行中。你可以使用crosstab，我不太熟悉，无法回答。或者你可以使用这样的粗略查询：

SELECT   e.*,
         (SELECT value FROM entry_fields AS ef WHERE name = 'command' AND ef.entryid = f.entryid) AS command,
         (SELECT value FROM entry_fields AS ef WHERE name = 'output' AND ef.entryid = f.entryid) AS output,
         (SELECT value FROM entry_fields AS ef WHERE name = 'code' AND ef.entryid = f.entryid) AS code,
         (SELECT value FROM entry_fields AS ef WHERE name = 'result' AND ef.entryid = f.entryid) AS result
FROM     entry AS e