How can I add text to SQL Column

You mean like this?:

SELECT 'www.mypizza.com/' + ColumnName AS ColumnName FROM TableName

Depending on the rest of your application environment, there is likely a much better way to accomplish this. But in terms of just using SQL to add static text to a column in a SELECT statement, you can just concatenate the text directly in the statement.

Or, if you wanted to UPDATE the column values, something like this:

UPDATE TableName SET ColumnName = 'www.mypizza.com/' + ColumnName

Same principle, just using an UPDATE instead of a SELECT, which will modify the underlying data instead of just modifying the view of the data.

If you are using MYSql, you can use the concat() as :

update tableName set columnName= CONCAT('www.mypizza.com/', columnName);

SQLFiddle

If you are using oracle you can use the concatenation operator '||' as :

update tableName set "columnName"='www.mypizza.com/'||"columnName";

SQLFiddle

In SQL Server you can use + for string concatenation as:

update tableName set name='www.mypizza.com/'+columnName;

SQLFiddle

You can simply update column using statement

update TableName set ColumnName  = 'www.mypizza.com/' + ColumnName  

OP doesn't specify which DBMS they are using. The following is for Postgres to update a text column by adding a prefix to it (tested with PostgreSQL v11):

UPDATE my_table 
SET column_1  = 'start_text_' || column_1
WHERE column_1 LIKE 'prefix_%'
;