作为
被接受的答案的延伸,Oracle在内部使用
ROW_NUMBER/RANK
函数。
OFFSET FETCH
语法只是一种语法糖。
可以通过使用
DBMS_UTILITY.EXPAND_SQL_TEXT
过程来观察:
准备样本:
CREATE TABLE rownum_order_test (
val NUMBER
);
INSERT ALL
INTO rownum_order_test
SELECT level
FROM dual
CONNECT BY level <= 10;
COMMIT;
查询:
SELECT val
FROM rownum_order_test
ORDER BY val DESC
FETCH FIRST 5 ROWS ONLY;
定期:
SELECT "A1"."VAL" "VAL"
FROM (SELECT "A2"."VAL" "VAL","A2"."VAL" "rowlimit_$_0",
ROW_NUMBER() OVER ( ORDER BY "A2"."VAL" DESC ) "rowlimit_$$_rownumber"
FROM "ROWNUM_ORDER_TEST" "A2") "A1"
WHERE "A1"."rowlimit_$$_rownumber"<=5 ORDER BY "A1"."rowlimit_$_0" DESC;
db<>fiddle演示
获取展开的SQL文本:
declare
x VARCHAR2(1000);
begin
dbms_utility.expand_sql_text(
input_sql_text => '
SELECT val
FROM rownum_order_test
ORDER BY val DESC
FETCH FIRST 5 ROWS ONLY',
output_sql_text => x);
dbms_output.put_line(x);
end;
/
WITH TIES
相当于用 RANK
函数展开:
declare
x VARCHAR2(1000);
begin
dbms_utility.expand_sql_text(
input_sql_text => '
SELECT val
FROM rownum_order_test
ORDER BY val DESC
FETCH FIRST 5 ROWS WITH TIES',
output_sql_text => x);
dbms_output.put_line(x);
end;
/
SELECT "A1"."VAL" "VAL"
FROM (SELECT "A2"."VAL" "VAL","A2"."VAL" "rowlimit_$_0",
RANK() OVER ( ORDER BY "A2"."VAL" DESC ) "rowlimit_$$_rank"
FROM "ROWNUM_ORDER_TEST" "A2") "A1"
WHERE "A1"."rowlimit_$$_rank"<=5 ORDER BY "A1"."rowlimit_$_0" DESC
和偏移量:
declare
x VARCHAR2(1000);
begin
dbms_utility.expand_sql_text(
input_sql_text => '
SELECT val
FROM rownum_order_test
ORDER BY val
OFFSET 4 ROWS FETCH NEXT 4 ROWS ONLY',
output_sql_text => x);
dbms_output.put_line(x);
end;
/
SELECT "A1"."VAL" "VAL"
FROM (SELECT "A2"."VAL" "VAL","A2"."VAL" "rowlimit_$_0",
ROW_NUMBER() OVER ( ORDER BY "A2"."VAL") "rowlimit_$$_rownumber"
FROM "ROWNUM_ORDER_TEST" "A2") "A1"
WHERE "A1"."rowlimit_$$_rownumber"<=CASE WHEN (4>=0) THEN FLOOR(TO_NUMBER(4))
ELSE 0 END +4 AND "A1"."rowlimit_$$_rownumber">4
ORDER BY "A1"."rowlimit_$_0"