oracle 12c新特性 FETCH FIRST、WITH TIES 关键字详解

几乎都是官方文档上的内容。

 

 

[ OFFSET offset { ROW | ROWS} ]

[ FETCH { FIRST | NEXT }[ { rowcount | percent PERCENT } ]

    { ROW| ROWS } { ONLY | WITH TIES } ]

 

row_limiting_clause

 

The row_limiting_clause allows you to limit therows returned by the query. You can

specify an offset, and number of rows or percentageof rows to return. You can use this

clause to implement top-N reporting. For consistentresults, specify the order_by_

clauseto ensure a deterministic sort order.

row_limiting_clause允许限制返回行的个数。

可以指定offset和行数（或者百分比）来返回行。

可以使用这个子句去实现top-N报表。

为保证一致性，需要指定order_by 子句以确定排列顺序。

 

 

 

OFFSET

Use this clause to specify the number of rows toskip before row limiting begins.

offset must be a number. If you specify a negativenumber, then offsetis treated as

0. If you specify NULL, or a number greater than orequal to the number of rows

returned by the query, then 0 rows are returned. Ifoffsetincludes a fraction, then the

fractional portion is truncated. If you do notspecify this clause, then offsetis 0 and

row limiting begins with the first row.

 

使用这个子句可以指定跳跃多少行开始计数。

offset必须为一个数字。

如果指定一个附属，那么会被当作0来处理。

如果指定为null，或者数字大于结果集的行数，就会返回0行。

如果offset是一个小数，那么小数点会被截取。

如果没有offset子句，那么默认为0，从第一行开始计数。

 

 

ROW | ROWS  

 

These keywords can be usedinterchangeably and are provided for

semantic clarity.

 

 

这些关键字使语义更加准确

 

 

FETCH

Use this clause to specify the number of rows orpercentage of rows to return. If you

do not specify this clause, then all rows arereturned, beginning at row offset+ 1.

 

使用这个子句去指定返回行的个数或者返回行的百分比。如果没有指定，那么所有的行都会被返回，开始行为offset+1。

 

 

FIRST | NEXT

These keywords can be used interchangeably and areprovided for

semantic clarity.

 

这些关键字使语义更加准确

 

 

rowcount| percent PERCENT  

 

Use rowcount to specify the number of rows toreturn.

rowcount must be a number. If you specify anegative number, then rowcountis

treated as 0. If rowcountis greater than the numberof rows available beginning at row

offset+ 1, then all available rows are returned. Ifrowcount includes a fraction, then

the fractional portion is truncated. If rowcountisNULL, then 0 rows are returned.

Use percent PERCENT to specify the percentage ofthe total number of selected rows to

return. percent must be a number. If you specify anegative number, then percentis

treated as 0. If percentis NULL, then 0 rows arereturned.

If you do not specify rowcountor percent PERCENT,then 1 row is returned.

 

使用rowcount去指定返回多少行。

rowcount必须为一个数字，如果指定了一个负数，那么rowcount会被当作0。如果rowcount大于以offset+1开始计数的所有行个数，那么所有的行都会被返回。

如果rowcount是一个小数，那么小数部分会被截断。如果rowcount为null，那么返回0行。

使用percent去指定返回总行数的百分比。必须为一个数字。如果指定为负数，那么会被当作0。

如果为null，那么返回0行。（其实都是一个套路嘛）

 

 

ROW | ROWS  

 

These keywords can be usedinterchangeably and are provided for

semantic clarity.

 

 

这些关键字使语义更加准确

 

 

ONLY | WITH TIES

Specify ONLYto return exactly the specified numberof rows or

percentage of rows.

 

指定only会返回明确的行数或者是百分比的行数。

Specify WITH TIES to return additional rows withthe same sort key as the last row

fetched. If you specify WITH TIES, then you mustspecify the order_by_clause. If you

do not specify the order_by_clause, then noadditional rows will be returned.

 

如果指定with ties子句，那么拥有和最后一行相同的排序键值的行都会被fetch。如果指定了with ties子句，那么必须指定order by 。如果没有指定order by，那么不会有附加的行被返回。

 

 

Restrictions on the row_limiting_clause

This clause is subject to thefollowing

restrictions:

 

■ You cannot specify this clause with the for_update_clause.

■ If you specify this clause, then the select list cannot contain thesequence

pseudocolumns CURRVALor NEXTVAL.

■ Materialized views are not eligible for an incremental refreshif the defining query

contains the row_limiting_clause.

row_limiting_clause子句的限制：

无法指定for update子句

无法包含序列的伪列currentval或者nextval

如果定义的查询语句中包含row_limiting_clause，那么无法在这之上创建增量刷新的物化视图。

下面为可能发生的错误举例：

 

1、

SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
OFFSET 5 ROWS FETCH first 5 ROWS ONLY for update ;

SELECT employee_id, last_name
*
ERROR at line 1:
ORA-02014: cannot select FOR UPDATE from view withDISTINCT, GROUP BY, etc.

2、

SELECT seq.currval,employee_id, last_name
FROM employees
ORDER BY employee_id
OFFSET 5 ROWS FETCH NEXT 5 ROWS ONLY
/

SELECT seq.currval,employee_id, last_name
           *
ERROR at line 1:
ORA-02287: sequence number not allowed here

 

3、

CREATE MATERIALIZED VIEW LOG ON employees withprimary key ;

Materialized view log created.

CREATE MATERIALIZED VIEW mym REFRESH FAST AS
(
SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
FETCH FIRST 5 ROWS ONLY
)
/

FETCH FIRST 5 ROWS ONLY
            *
ERROR at line 6:
ORA-12015: cannot create a fast refresh materializedview from a complex query

CREATE MATERIALIZED VIEW mym REFRESH FAST AS
SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
/

Materialized view created.

CREATE MATERIALIZED VIEW mym
as
SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
FETCH FIRST 5 ROWS ONLY
/

FETCH FIRST 5 ROWS ONLY
*
ERROR at line 6:
ORA-00933: SQL command not properly ended

记住加括号

CREATE MATERIALIZED VIEW mym
as
(SELECTemployee_id, last_name
FROM employees
ORDER BY employee_id
FETCH FIRST 5 ROWS ONLY)
/

Materialized view created.

默认是按需更新的物化视图所以没有什么问题。

先来几个例子

Row Limiting: Examples

The following statement returns the 5 employeeswith the
lowest employee_id values:

 

下面返回的是empid最小的5行。

SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
FETCH FIRST 5 ROWS ONLY;

 

这里将FIRST换成NEXT或者将ROWS换成ROW都没有什么区别，但是拥有这些关键字是必须的。也证明了官档上说的它们的作用是让语义更加准确。

EMPLOYEE_ID LAST_NAME

----------- -------------------------

100 King

101 Kochhar

102 De Haan

103 Hunold

104 Ernst

下面返回的是跳过empid最小的5行的下5行数据。

SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
OFFSET 5 ROWS FETCH NEXT 5 ROWS ONLY;

offset 不提供percent功能…

EMPLOYEE_ID LAST_NAME

----------- -------------------------

105 Austin

106 Pataballa

107 Lorentz

108 Greenberg

109 Faviet

下面返回的是薪水最小的5%的数据。

SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
OFFSET 5 ROWS FETCH NEXT 5 ROWS ONLY;

EMPLOYEE_ID LAST_NAME SALARY

----------- ------------------------- ----------

132 Olson 2100

128 Markle 2200

136 Philtanker 2200

127 Landry 2400

135 Gee 2400

119 Colmenares 2500

 使用了with ties子句，下面的语句返回最小薪水的5%的数据，附加和最后一行相同薪水的数据。

 

SELECT employee_id, last_name
FROM employees
ORDER BY employee_id
OFFSET 5 ROWS FETCH NEXT 5 ROWS ONLY;

EMPLOYEE_ID LAST_NAME SALARY

----------- ------------------------- ----------

132 Olson 2100

128 Markle 2200

136 Philtanker 2200

127 Landry 2400

135 Gee 2400

119 Colmenares 2500

131 Marlow 2500

140 Patel 2500

144 Vargas 2500

182 Sullivan 2500

191 Perkins 2500

 

如果通过上面的例子还没有完全搞懂，那么就看下面的官方文档的翻译吧。

Perform top-N queries by specifying an offset, andthe number of rows or

percentage of rows to return.

 

可以通过指定偏移量、需要返回的行数来实现top-n查询