实用SQL語句 .

1、将一个表中的内容拷贝到另外一个表中
insert into testT1(a1,b1,c1) select a,b,c from test;
insert into testT select * from test; (前提是兩個表的結構完全相同)
 
insert into notebook(id,title,content)
select notebook_sequence.NEXTVAL,first_name,last_name from students;
 
注:a1,b1,c1是現在表中的字段名。A,b,c是原表中的字段名
拷贝单条数据
insert into a (select '123' as id, temp,temp2 from b where id=10)
2、查找重复记录
SELECT DRAWING,DSNO FROM EM5_PIPE_PREFAB
WHERE ROWID!=(SELECT MAX(ROWID) FROM EM5
_PIPE_PREFAB D
WHERE EM5_PIPE_PREFAB.DRAWING=D.DRAWING AND
EM5_PIPE_PREFAB.DSNO=D.DSNO);
3、删除重复记录
DELETE FROM EM5_PIPE_PREFAB
WHERE ROWID!=(SELECT MAX(ROWID) FROM EM5
_PIPE_PREFAB D
WHERE EM5_PIPE_PREFAB.DRAWING=D.DRAWING AND
EM5_PIPE_PREFAB.DSNO=D.DSNO);
4、復制表結構
    在ORACLE中: create table new_table as select * from old_table where rownum < 1;
在SQL SERVER中:说明：复制表(只复制结构,源表名：a 新表名：b)

　　SQL: select * into b from a where 1<>1 　
 
语句
5、一次刪除多個字段
alter table TEST_A drop (TEST_num,test_name);
6、實現一個表的備份
create table tableName_bak as select * from tableName;
7、創建兩個表的主外鍵關聯
 
--先創建主表
create table t1(id int,stu_name varchar(50));
--為主表增加持久的主鍵約束
alter table t1 add(constraint t1_id primary key(id) deferrable);
--創建從表，并將從表的id設為主鍵
create table t2(id int primary key,sex varchar(10),address varchar(50));
--為從表增加外鍵約束，該約束來自于主表所創建的約束字段
--當從主表中刪除記錄時，自動刪除從表中與之相對的具有相同id的記錄
--當在從表中插入記錄時，不能夠插入在主表中沒有id的記錄
alter table t2 add(constraint t2_id_fk foreign key(id) references t1(id) on delete cascade deferrable);
--測試數據
insert into t1 values(1,'FLB');
insert into t1 values(2,'FLB1');
insert into t1 values(3,'FLB2');
insert into t1 values(4,'FLB3');
 
insert into t2 values(1,'boy','leijiang');
insert into t2 values(2,'boy','leijiang');
insert into t2 values(3,'boy','leijiang');
insert into t2 values(4,'boy','leijiang');
--下面這條記錄插入錯誤
insert into t2 values(5,'boy','leijiang');
--刪除主表中的記錄
delete t1;
--查看從表中記錄，已經沒有記錄，說明約束成功
select * from t2;
 
 
8、關于系統時間
--取得當前系統時間、當前月的最后一天，離當前月的結束還有几天
SELECT SYSDATE, LAST_DAY(SYSDATE) "Last",
LAST_DAY(SYSDATE) - SYSDATE "Days Left"
FROM DUAL;
--取得當前星期几
select to_char(sysdate,'Day') from dual;
--取得當前月
select to_char(sysdate,'Month') from dual;
--取得指定日期的星期
select to_char(to_date('20020126','yyyymmdd'),'Day') from dual;