How can I use a variable on BigQuery SQL Editor? - google-bigquery

Question

While writing SQL queries on BigQuery UI, sometimes I do a lot of JOIN over multiple tables using many WHERE clauses with the same date condition for each table.
Whenever I need to see the results for a different date, I have to replace it at multiple locations in the query. I wonder if there is a simple way to use a variable in the BQ SQL Editor and pass it just once (top/bottom)?
This is true for all the complicated queries as we have to search throughout the query for variables to change.

Answer 1

While parameterized queries are not available in the Console. You can use Scripting, instead.
According to your need, you can use DECLARE and/or SET. It is stated in the documentation that:
DECLARE: Declares a variable of the specified type. If the DEFAULT clause is specified, the variable is initialised with the
value of the expression; if no DEFAULT clause is present, the variable
is initialised with the value NULL
The syntax is as follows:
#Declaring the variable's type and initialising the variable using DEFAULT
DECLARE variable STRING DEFAULT 'San Francisco';
SELECT * FROM `bigquery-public-data.san_francisco_bikeshare.bikeshare_regions`
#using the variable inside the where clause
WHERE name = variable
SET: Sets a variable to have the value of the provided expression, or sets multiple variables at the same time based on the
result of multiple expressions.
And the syntax, as below:
#Declaring the variable using Declare and its type
DECLARE variable STRING;
#Initialising the variable
SET variable = 'San Francisco';
SELECT * FROM `bigquery-public-data.san_francisco_bikeshare.bikeshare_regions`
#using the variable inside the where clause
WHERE name = variable
In both examples above, I have queried against a public dataset bigquery-public-data.san_francisco_bikeshare.bikeshare_regions. Also, both outputs are the same,
Row region_id name
1 3 San Francisco
In addition to the above example, more specifically to your case, when declaring a variable as date you can to it as follows:
DECLARE data1 DATE DEFAULT DATE(2019,02,15);
WITH sample AS(
SELECT DATE(2019,02,15) AS date_s, "Some other field!" AS string UNION ALL
SELECT DATE(2019,02,16) AS date_s, "Some other field!" AS string UNION ALL
SELECT DATE(2019,02,17) AS date_s, "Some other field!" AS string UNION ALL
SELECT DATE(2019,02,18) AS date_s, "Some other field!" AS string
)
SELECT * FROM sample
WHERE date_s = data1
And the output,
Row date_s string1
1 2019-02-15 Some other field!