Column Reference

A column object belongs to a table and holds a sequence of data. To refer to a column object, we can use <table>.<column> if the statement is not a SQL statement. Please note that <table>.<column> is read only and cannot be used to modify a column.

$ t=table(2 3 4 as x, 5 6 7 as y);
$ t;

x

y

2

5

3

6

4

7

$ t.x;
[2,3,4]

$ a=t.x;
$ a+=1;
$ a;
[3,4,5]

$ t.x;
[2,3,4]
// column t.x is unchanged

$ t.x+=1;
Syntax Error: [line #1] Can't use assignment in expression.
// this is because t.x is read only. To modify column x, use t[`x] or update clause.

In a SQL statement, the table name can be ignored in front of the variable name.

select x from t;

x

2

3

4

How does the system tell if a variable is column reference, a variable name or a function name? This problem is dynamically disambiguated at runtime with the order of column reference, variable name, function name. Specifically, in a SQL statement, if a table column and a defined variable have the same name, the system always interprets the name as the table column. If we want to refer the defined variable in a SQL statement, we have to rename the variable. If a function has the same name as a table column or a variable and we want to use the function name in a SQL statement, we can use an “&” or module name before the function name to qualify it.

$ x=1..3
$ z=7..9
$ t1=select x, y, z from t;

$ t1;
// since table t has column x, the system interprets x as t.x in the sql statement although there is a separately defined variable x.
// although z is not in t1, the system recognizes it is of the same size as t.x and t.y. Therefore the system uses z together with t.x and t.y to make a new table t1.

x

y

z

2

5

7

3

6

8

4

7

9

$ avg=12.5
$ t1=table(1 2 2 3 3 as id, 1.5 1.8 3.2 1.7 2.5 as value)
$ select * from t1 where value > contextby(&avg,value,id);

id

value

2

3.2

3

2.5

$ select * from t1 where value > contextby(::avg,value,id);

id

value

2

3.2

3

2.5

$ select * from t1 where value > contextby(avg,value,id);
To use template 'contextby', the first argument must be a function definition

$ t=table(1 2 3 as ID, 4 5 6 as check)

$ check=1
$ check1=10
$ def check(x):x*10
$ def check1(x):x*100
$ def check2(x,y):x+2*y;

$ t;

ID

check

1

4

2

5

3

6

$ select ID+check from t;
// column ID plus column check from table t

add_ID

5

7

9

$ select ID+check1 from t;
// column ID plus the variable of check1

add_ID

11

12

13

$ select accumulate(check2, check) as y from t;
// apply the accumulate template on the function definition of check2 and the column check from table t

y

4

14

26