Updated May 15, 2023
Definition of Redshift coalesce
Redshift coalesce function will return the first not null value as result, coalesce function is built-in function of redshift. Coalesce function is work same as IFNULL function in SQL, this function will evaluates the argument from left to right and after finding the not null values it will not evaluate the remaining arguments. This function will accept the number of arguments but it will returns the first argument which was not null, if all the value are null then it will return the empty result set. This function is useful when we have returning the backup value.
Syntax:
Below is the syntax of coalesce function in redshift are as follows.
1) COALESCE (Value1, Value2,….,ValueN);
2) COALESCE (Argument1, Argument2,…., ArgumentN);
3) Select column1, column2,…., COALESCE (column_name1, column_name2)
From table_name;
Parameters
1) Value1 to ValueN – Any value which we have using with coalesce function in redshift. For multiple values coalesce function will return a first not-null value in result.
2) Coalesce – Coalesce is the function name in redshift which returns first not-null value as a result. Coalesce function is also known as NVL function in redshift.
3) Select – Select is used to fetch data from table by using a coalesce function in redshift. We can use multiple column or single column at one time to fetch data from table.
4) Argument 1 to Argument N – Argument is nothing but an integer value that we have passing with coalesce function in redshift. If suppose we have passing five argument with coalesce function and if first argument contains null value then coalesce function will return next not null value.
5) Column name – This is a column name of table which was we have using with coalesce function. If we want to fetch data from table using coalesce function in redshift then we need to pass column name.
6) Table name – This is nothing but the name of table which was we have using with coalesce function to retrieve first not-null value.
How coalesce works in Redshift?
- Redshift coalesce function is used to return the first not null value from table column or argument list which was we have used in query.
- This function will evaluates values or arguments from left to right to find the not-null value or argument. After finding first argument it will not evaluating the remaining argument from column or values which was used in query.
- If our table column contains multiple null value but in result we have not required any null value at that time we have using coalesce function in redshift.
- Coalesce function will returns the empty result when we have passing all the null values in our query.
- Below example shows that coalesce function will returns the empty result when we have passing all the null values.
Code:
Select coalesce (NULL, NULL, NULL, NULL, NULL);
- In above example, we can see that we have used all argument as null values. After checking all the null values result is showing the empty set.
- After finding first not null value coalesce function will stop the query execution it will not check other argument or values.
- Below example shows that after finding first not null value coalesce function will stop the query execution.
Code:
Select coalesce (NULL, NULL, 10, NULL, 50);
- In above example, we can see that the result of argument list (NULL, NULL, 10, NULL, 50) is 10. After finding 10 coalesce function will not search the next value, it will not goes to search next 50 values which was not null. It will stop evaluation at 10 value.
- Coalesce function is same function as NVL function which was used in big query. At the time of migrating the code into the redshift we need to rewrite this function into big query.
- We can use single as well as multiple column while evaluating data using coalesce function in redshift.• If we want query which was return the result of null values of certain function same time we have using coalesce in redshift.
- Coalesce function in redshift is also used with argument list which was we have using in our query. It will shows first not null values using argument list.
Examples
Below example shows coalesce in redshift are as follows.
1) Coalesce function with argument list –
Below example shows that coalesce function with argument list are as follows. We have passing the (NULL, NULL, 15, NULL, 20, NULL, 25, 30) argument list with coalesce function.
The result of the argument is 15. We can see that it will retrieves first not null value as 15 and skipping all the values which was comes after 15.
Code:
Select coalesce (NULL, NULL, 15, NULL, 20, NULL, 25, 30);
2) Coalesce function with single argument
- Below example shows that coalesce function with single argument. We have passing only one argument as 50.
- The result is below example is 50, because coalesce function has finds not null value in first argument.
Code:
Select coalesce (50);
3) Coalesce function using single column as argument list –
- Below example shows that coalesce function using a single column as argument list are as follows.
- We have using the start_date column from date table.
Code:
Select coalesce (start_date) from date1 order by 1;
4) Coalesce function using multiple column as argument list –
- Below example shows that coalesce function using multiple column as argument list are as follows.
- We have using the start_date and end_date column from date table.
Code:
Select coalesce (start_date, end_date) from date1 order by 1;
Conclusion
Redshift coalesce function is very useful to find the first not null value from column or argument list. If suppose all column value contains the not null value then coalesce function will returns the empty result. Coalesce function will stop execution after finding the first not null value.
Recommended Articles
This is a guide to Redshift coalesce. Here we discuss Definition, syntax, How coalesce works in Redshift? examples with code implementation respectively. You may also have a look at the following articles to learn more –
