oracle 分組函式與group by正確用法詳解與詳細解釋
阿新 • • 發佈:2019-01-30
oracle資料庫 分組函式與group by正確用法詳解與詳細解釋
1.查詢時同時查詢了分組函式列和非分組函式列就需要使用group by,但是僅僅查詢分組函式列可以不結合group by使用。
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024) from dba_data_files;
ERROR at line 1:
ORA-00937: not a single-group group function
not a single-group group function報錯原因是:用了分組函式,除了分組以外的列都要寫在group by子句中;
加入 group by後不再報錯:
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024) from dba_data_files group by tablespace_name;
TABLESPACE_NAME SUM(USER_BYTES/1024/1024)
------------------------------ -------------------------
SYSAUX 459
UNDOTBS1 69
USERS 4
SYSTEM 799
2.如果不是把除了分組以外的所有列寫到group by子句仍然會報錯:
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024), FILE_ID from dba_data_files group by tablespace_name;
select TABLESPACE_NAME,sum(user_bytes/1024/1024), FILE_ID from dba_data_files group by tablespace_name *
ERROR at line 1:
ORA-00979: not a GROUP BY expression
下面把除了分組以外的所有列寫到group by子句,就可以正常查詢了:
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024), FILE_ID from dba_data_files group by tablespace_name,FILE_ID;
TABLESPACE_NAME SUM(USER_BYTES/1024/1024) FILE_ID
------------------------------ ------------------------- ----------
USERS 4 7
SYSAUX 459 3
UNDOTBS1 69 4
SYSTEM 799 1
3.group by子句一定要與分組函式結合使用,否則沒有意義。
SQL> select TABLESPACE_NAME,user_bytes/1024/1024 from dba_data_files group by tablespace_name;
ERROR at line 1:
ORA-00979: not a GROUP BY expression
4.group by 子句中的列不必包含在SELECT列表中,
SQL> select sum(user_bytes/1024/1024) from dba_data_files group by file_id;
SUM(USER_BYTES/1024/1024)
-------------------------
749
549
4
312.125
104
但是如果查詢列中出現了其他非分組函式列就會報錯:
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024) from dba_data_files group by file_id;
ERROR at line 1:
ORA-00979: not a GROUP BY expression
===============關於分組函式的介紹:
分組函式的語法:
SELECT [column,] group_function(column), ...
FROM table
[WHERE condition]
[GROUP BY column]
[ORDER BY column];
分組函式作用於一組資料,並對一組資料返回一個值.
常見的分組函式有:
count 返回找到的記錄數
min 返回一個數字列或計算列的最小值
max 返回一個數字列或計算列的最大值
sum 返回一個數字列或計算列總和
1.查詢時同時查詢了分組函式列和非分組函式列就需要使用group by,但是僅僅查詢分組函式列可以不結合group by使用。
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024) from dba_data_files;
ERROR at line 1:
ORA-00937: not a single-group group function
not a single-group group function報錯原因是:用了分組函式,除了分組以外的列都要寫在group by子句中;
加入 group by後不再報錯:
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024) from dba_data_files group by tablespace_name;
TABLESPACE_NAME SUM(USER_BYTES/1024/1024)
------------------------------ -------------------------
SYSAUX 459
UNDOTBS1 69
USERS 4
SYSTEM 799
2.如果不是把除了分組以外的所有列寫到group by子句仍然會報錯:
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024), FILE_ID from dba_data_files group by tablespace_name;
select TABLESPACE_NAME,sum(user_bytes/1024/1024), FILE_ID from dba_data_files group by tablespace_name *
ERROR at line 1:
ORA-00979: not a GROUP BY expression
下面把除了分組以外的所有列寫到group by子句,就可以正常查詢了:
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024), FILE_ID from dba_data_files group by tablespace_name,FILE_ID;
TABLESPACE_NAME SUM(USER_BYTES/1024/1024) FILE_ID
------------------------------ ------------------------- ----------
USERS 4 7
SYSAUX 459 3
UNDOTBS1 69 4
SYSTEM 799 1
3.group by子句一定要與分組函式結合使用,否則沒有意義。
SQL> select TABLESPACE_NAME,user_bytes/1024/1024 from dba_data_files group by tablespace_name;
ERROR at line 1:
ORA-00979: not a GROUP BY expression
4.group by 子句中的列不必包含在SELECT列表中,
SQL> select sum(user_bytes/1024/1024) from dba_data_files group by file_id;
SUM(USER_BYTES/1024/1024)
-------------------------
749
549
4
312.125
104
但是如果查詢列中出現了其他非分組函式列就會報錯:
SQL> select TABLESPACE_NAME,sum(user_bytes/1024/1024) from dba_data_files group by file_id;
ERROR at line 1:
ORA-00979: not a GROUP BY expression
===============關於分組函式的介紹:
分組函式的語法:
SELECT [column,] group_function(column), ...
FROM table
[WHERE condition]
[GROUP BY column]
[ORDER BY column];
分組函式作用於一組資料,並對一組資料返回一個值.
常見的分組函式有:
count 返回找到的記錄數
min 返回一個數字列或計算列的最小值
max 返回一個數字列或計算列的最大值
sum 返回一個數字列或計算列總和
avg 返回一個數字列或計算列的平均值
================關於group by子句