使用十进制值更新 SQL 表只会产生 0

Question

On MSSQL: I am adding a column to a table

ALTER table
ADD x_column DECIMAL

Now, I perform a calculation using another column of the same table:

SELECT SELECT CAST(a_column AS decimal)/CAST (1000000000*0.1*12 AS decimal) FROM table

This yields the expected result:

+---+-----------------------+
¦ 1 ¦ 0.0000732950000000000 ¦
+---+-----------------------+
¦ 2 ¦ 0.0000016912500000000 ¦
+---+-----------------------+
¦ 3 ¦ 0.0005905541666666666 ¦
+---+-----------------------+                  

However, when I try to update the table with these values, all I get are zeroes:

UPDATE table
SET x_column = CAST(a_column AS DECIMAL)/CAST(1000000000*0.1*12 AS decimal)

 id   x_column
+---+-----------------------+
¦ 1 ¦ 0                     ¦
+---+-----------------------+
¦ 2 ¦ 0                     ¦
+---+-----------------------+
¦ 3 ¦ 0                     ¦
+---+-----------------------+ 

What I'd like is for the results of the above SELECT to be inserted into the column in the table.

Answer 1

You forgot to add the precision and scale on the DECIMAL datatype .

If not specified, the default scale is 0.

When you add your column you should use:

ALTER table
ADD x_column DECIMAL(35, 19)

or something larger up to 38 digits.

Precision is how many digits your number would have, while the scale is the number of digits after the decimal point.

Answer 2

You should add scale and precision to your columne declaration, if omitted it is 0

ALTER table ADD x_column DECIMAL(28,12)