在一行中从 table 中选择 2 行的最佳方法是什么？

Question

我有一个table喜欢关注

TABLE_A
ID           PERSON_ID    NAME         GRADE
----------   ----------   ----------   ----------
1            1            NAME_1       10
2            1            NAME_1       20
3            2            NAME_2       30
4            2            NAME_2       40
...

在这个table中，每个名字正好有两行（两个年级）。我想查询结果如下

RESULT
PERSON_ID    NAME         GRADE1       GRADE_2
----------   ----------   ----------   ----------
1            NAME_1       10            20
2            NAME_2       30            40

最好的方法是什么。我可以使用自连接，但我认为这不是正确的方法

Answer 1

为 SQL 服务器尝试此操作

; WITH CTE AS(
SELECT PERSON_ID, NAME, GRADE AS GRADE_1, 
LEAD(GRADE) OVER(PARTITION BY NAME ORDER BY NAME) AS GRADE_2 
FROM TABLE_A
) 
SELECT * FROM CTE 
WHERE GRADE_2 IS NOT NULL

RESULT SET:

PERSON_ID    NAME         GRADE_1       GRADE_2
----------   ----------   ----------   ----------
1            NAME_1       10            20
2            NAME_2       30            40

Answer 2

如果你使用的是MySQL，并且所有的名字正好有两条记录，一年级是id较低的那个，二年级是另一个，那么你可以使用这样的查询一：

select
  person_id,
  name,
  group_concat(grade order by id) as grades
from
  table_a
group by
  person_id,
  name

然后，如果您想将成绩分成两个新列，您可以使用 SUBSTRING_INDEX:

select
  person_id,
  name,
  substring_index(group_concat(grade order by id), ',', 1) as grade1,
  substring_index(group_concat(grade order by id), ',', -1) as grade2
from
  table_a
group by
  person_id,
  name

Answer 3

你可以按照对方的建议使用GROUP BY。

或者您可以加入。

select t1.person_id, t1.grade as grade1, t2.grade as grade2
from TABLE_A t1 join TABLE_A t2 on t1.person_id=t2.person_id and t1.id!=t2.id

此 JOIN 连接同一个人的所有行，但不连接具有相同 ID 的行，因此您可以过滤掉重复项。

Answer 4

一旦话题发起人没有提到成绩的顺序，"there is exactly two rows (two grades)"我认为最简单的方法：

SELECT PERSON_ID, 
       MAX(NAME) as NAME, 
       MIN(GRADE) as GRADE1, 
       MAX(GRADE) as GRADE2
 FROM TABLE_A
 GROUP BY PERSON_ID

Answer 5

我在这个 link 中找到了 PostgreSQL 的最佳答案 https://www.postgresql.org/docs/9.2/static/tablefunc.html 在 PostgreSQL 中有 crosstab(text) 函数。

The crosstab function is used to produce "pivot" displays, wherein data is listed across the page rather than down. For example, we might have data like

row1    val11
row1    val12
row1    val13
...
row2    val21
row2    val22
row2    val23
...

which we wish to display like

row1    val11   val12   val13   ...
row2    val21   val22   val23   ...
...

The crosstab function takes a text parameter that is a SQL query producing raw data formatted in the first way, and produces a table formatted in the second way. The sql parameter is a SQL statement that produces the source set of data. This statement must return one row_name column, one category column, and one value column. N is an obsolete parameter, ignored if supplied (formerly this had to match the number of output value columns, but now that is determined by the calling query). For example, the provided query might produce a set something like:

row_name    cat    value
----------+-------+-------
  row1      cat1    val1
  row1      cat2    val2
  row1      cat3    val3
  row1      cat4    val4
  row2      cat1    val5
  row2      cat2    val6
  row2      cat3    val7
  row2      cat4    val8

The crosstab function is declared to return setof record, so the actual names and types of the output columns must be defined in the FROM clause of the calling SELECT statement, for example:

SELECT * FROM crosstab('...') AS ct(row_name text, category_1 text, category_2 text);

This example produces a set something like:

           <== value  columns  ==>
 row_name   category_1   category_2
----------+------------+------------
  row1        val1         val2
  row2        val5         val6

有关更多信息，请阅读 F.35.1.2. 部分 URL 进入答案顶部

在一行中从 table 中选择 2 行的最佳方法是什么？

What is the best way for selecting 2 row from table in one row?

mysql

sql

postgresql

crosstab