where

where子句用来选择满足指定条件的记录。

例子

$ sym = `C`MS`MS`MS`IBM`IBM`C`C`C$SYMBOL
$ price= 49.6 29.46 29.52 30.02 174.97 175.23 50.76 50.32 51.29
$ qty = 2200 1900 2100 3200 6800 5400 1300 2500 8800
$ timestamp = [09:34:07,09:36:42,09:36:51,09:36:59,09:32:47,09:35:26,09:34:16,09:34:26,09:38:12]
$ t1 = table(timestamp, sym, qty, price);
$ t1;

timestamp

sym

qty

price

09:34:07

C

2200

49.6

09:36:42

MS

1900

29.46

09:36:51

MS

2100

29.52

09:36:59

MS

3200

30.02

09:32:47

IBM

6800

174.97

09:35:26

IBM

5400

175.23

09:34:16

C

1300

50.76

09:34:26

C

2500

50.32

09:38:12

C

8800

51.29

使用含有一个条件的where子句:

select * from t1 where sym=`IBM;

timestamp

sym

qty

price

09:32:47

IBM

6800

174.97

09:35:26

IBM

5400

175.23

select * from t1 where timestamp.minute()>=09:36m;

timestamp

sym

qty

price

09:36:42

MS

1900

29.46

09:36:51

MS

2100

29.52

09:36:59

MS

3200

30.02

09:38:12

C

8800

51.29

在where子句中,"=="等于"="。

select * from t1 where sym==`IBM;

timestamp

sym

qty

price

09:32:47

IBM

6800

174.97

09:35:26

IBM

5400

175.23

使用含有多个条件的where子句:

select * from t1 where sym=`IBM and qty>=2000 or timestamp>09:37:00;

timestamp

sym

qty

price

09:32:47

IBM

6800

174.97

09:35:26

IBM

5400

175.23

09:38:12

C

8800

51.29

"and", "&&" 和 "," 表示必须同时满足用该符号连接的条件。

select * from t1 where qty>=2000, timestamp.minute()>=09:36m;

timestamp

sym

qty

price

09:36:51

MS

2100

29.52

09:36:59

MS

3200

30.02

09:38:12

C

8800

51.29

select * from t1 where qty>=2000 and timestamp.minute()>=09:36m;

timestamp

sym

qty

price

09:36:51

MS

2100

29.52

09:36:59

MS

3200

30.02

09:38:12

C

8800

51.29

select * from t1 where qty>=2000 && timestamp.minute()>=09:36m;

timestamp

sym

qty

price

09:36:51

MS

2100

29.52

09:36:59

MS

3200

30.02

09:38:12

C

8800

51.29

where条件中的函数和模板:

下面的例子中,我们将价格高于平均价格的记录选择出来。这里对表中所有记录使用了函数avg。

select * from t1 where price>avg(price);

timestamp

sym

qty

price

09:32:47

IBM

6800

174.97

09:35:26

IBM

5400

175.23

要计算每个股票的平均价格,可以使用 c/contextby。下面的例子查询每个股票的价格大于平均价格的记录。

select * from t1 where price>contextby(avg, price, sym) order by sym, price;

timestamp

sym

qty

price

09:34:16

C

1300

50.76

09:38:12

C

8800

51.29

09:35:26

IBM

5400

175.23

09:36:59

MS

3200

30.02

要随机抽取分区作为样本,只需在where子句中使用 sample 函数即可。

n=1000000
ID=rand(50, n)
x=rand(1.0, n)
t=table(ID, x)
db=database("C:/DolphinDB/Data/rangedb1", RANGE,  0 10 20 30 40 50)
pt = db.createPartitionedTable(t, `pt, `ID)
pt.append!(t)
pt=loadTable(db,`pt);

pt有5个分区,随机抽取两个分区的数据作为样本,可以使用下面的语句:

x = select * from pt where sample(ID, 0.4);
x = select * from pt where sample(ID, 2);