distinct的只显示一次重复出更的值。
不管这个值出现多少次只显示一次。
distinct 字段名1，字段名2 from 表格 order by 字段名1
最好和order by 结合使用。可以提高效率
SQL> SQL> CREATE TABLE employees ( 2 au_id CHAR(3) NOT NULL, 3 au_fname VARCHAR(15) NOT NULL, 4 au_lname VARCHAR(15) NOT NULL, 5 phone VARCHAR(12) NULL , 6 address VARCHAR(20) NULL , 7 city VARCHAR(15) NULL , 8 state CHAR(2) NULL , 9 zip CHAR(5) NULL 10 ); Table created. SQL> SQL> INSERT INTO employees VALUES('A01','S','B','111-111-1111','75 St','Boston','NY','11111'); 1 row created. SQL> INSERT INTO employees VALUES('A02','W','H','222-222-2222','2922 Rd','Boston','CO','22222'); 1 row created. SQL> INSERT INTO employees VALUES('A03','H','H','333-333-3333','3800 Ave, #14F','San Francisco','CA','33333'); 1 row created. SQL> INSERT INTO employees VALUES('A04','K','H','444-444-4444','3800 Ave, #14F','San Francisco','CA','44444'); 1 row created. SQL> INSERT INTO employees VALUES('A05','C','K','555-555-5555','114 St','New York','NY','55555'); 1 row created. SQL> INSERT INTO employees VALUES('A06',' ','K','666-666-666','390 Mall','Palo Alto','CA','66666'); 1 row created. SQL> INSERT INTO employees VALUES('A07','P','O','777-777-7777','1442 St','Sarasota','FL','77777'); 1 row created. SQL> SQL> SQL> SELECT DISTINCT state 2 FROM employees;
看过滤重复的sql语句
SQL> select * from Employee
2 /ID FIRST_NAME LAST_NAME START_DAT END_DATE SALARY CITY DESCRIPTION
---- ---------- ---------- --------- --------- ---------- ---------- ---------------
01 Jason Martin 25-JUL-96 25-JUL-06 1234.56 Toronto Programmer
02 Alison Mathews 21-MAR-76 21-FEB-86 6661.78 Vancouver Tester
03 James Smith 12-DEC-78 15-MAR-90 6544.78 Vancouver Tester
04 Celia Rice 24-OCT-82 21-APR-99 2344.78 Vancouver Manager
05 Robert Black 15-JAN-84 08-AUG-98 2334.78 Vancouver Tester
06 Linda Green 30-JUL-87 04-JAN-96 4322.78 New York Tester
07 David Larry 31-DEC-90 12-FEB-98 7897.78 New York Manager
08 James Cat 17-SEP-96 15-APR-02 1232.78 Vancouver Tester8 rows selected.
SQL>
SQL>
SQL> SELECT Description FROM employee
2 /DESCRIPTION
---------------
Programmer
Tester
Tester
Manager
Tester
Tester
Manager
Tester8 rows selected.
SQL> SELECT DISTINCT Description FROM employee
2 /DESCRIPTION
---------------
Programmer
Manager
Tester
如果要处理多列表的话，字段以“,"分开就要以了，如下
SQL> SELECT DISTINCT city, state
2 FROM employees
关于增加了distinct后查询的效率测试
只有增加DISTINCT关键字，Oracle必然需要对后面的所有字段进行排序。以前也经常发现由于开发人员对SQL不是很理解，在SELECT列表的20多个字段前面添加了DISTINCT，造成查询基本上不可能执行完成，甚至产生ORA-7445错误。所以一直向开发人员强调DISTINCT给性能带来的影响。
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没想到开发人员在测试一条大的SQL的时候，告诉我如果加上了DISTINCT，则查询大概需要4分钟左右可以执行完，如果不加DISTINCT，则查询执行了10多分钟，仍然得不到结果。
首先想到的是可能DISTINCT是在子查询中，由于加上了DISTINCT，将第一步结果集缩小了，导致查询性能提高，结果一看SQL，发现DISTINCT居然是在查询的最外层。
由于原始SQL太长，而且牵扯的表太多，很难说清楚，这里模拟了一个例子，这个例子由于数据量和SQL的复杂程度限制，无法看出二者执行时间上的明显差别。这里从两种情况的逻辑读对比来说明问题。
首先建立模拟环境：
SQL> CREATE TABLE T1 AS SELECT * FROM DBA_OBJECTS
2 WHERE OWNER = 'SYS'
3 AND OBJECT_TYPE NOT LIKE '%BODY'
4 AND OBJECT_TYPE NOT LIKE 'JAVA%';
Table created.
SQL> CREATE TABLE T2 AS SELECT * FROM DBA_SEGMENTS WHERE OWNER = 'SYS';
Table created.
SQL> CREATE TABLE T3 AS SELECT * FROM DBA_INDEXES WHERE OWNER = 'SYS';
Table created.
SQL> ALTER TABLE T1 ADD CONSTRAINT PK_T1 PRIMARY KEY (OBJECT_NAME);
Table altered.
SQL> CREATE INDEX IND_T2_SEGNAME ON T2(SEGMENT_NAME);
Index created.
SQL> CREATE INDEX IND_T3_TABNAME ON T3(TABLE_NAME);
Index created.
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(USER, 'T1', METHOD_OPT => 'FOR ALL INDEXED COLUMNS SIZE 100', CASCADE => TRUE)
PL/SQL procedure successfully completed.
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(USER, 'T2', METHOD_OPT => 'FOR ALL INDEXED COLUMNS SIZE 100', CASCADE => TRUE)
PL/SQL procedure successfully completed.
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(USER, 'T3', METHOD_OPT => 'FOR ALL INDEXED COLUMNS SIZE 100', CASCADE => TRUE)
PL/SQL procedure successfully completed.
