![]() ![]() Now you could not JOIN in Genre Names, How could we replace the GenreId in the Track table?ĬASE WHEN allows you to assign the genres yourself. Let’s say you lost the “Genre” table (indicated with the red X through it). However, the “GenreId” column is a reference to another table that links each id number to a genre of music. The “TrackId” in the table below is unique for each entry. The “Track” table in the Chinook database is a large, informational table on many different songs by many different artists. This example uses the Chinook database with PostgreSQL 11. To determine what data type is used in a column you can use the “\d (Table name)” command and look for the “Type” column: \d "Track" Remember you can cast any of your values in order to make the data types match. The Name field is a VARCHAR and cannot be compared to an INTEGER. Name = 40 ) THEN 'Rock' END AS Genre FROM Track ORDER BY Track. If you try to compare incompatible types SQL will return an “Invalid Input Syntax” error. Make sure that you are comparing the correct data types in your queries. More on optimizing queries from Data School here. If queries are taking a long time to finish, consider finding ways to optimize your query. This can dramatically increase the query time. CASE WHEN queries can become very slow because the query has to check each condition for every row until it finds a case where it satisfies the condition. There is no defined maximum for the number of WHEN conditions you can have within a CASE WHEN statement. Time to complete a query can be a problem. Note: you do not have to show all of the columns, or any columns besides the CASE column in the output. SELECT ( optional : any desired columns ), CASE WHEN ( condition ) THEN ( desired output ) WHEN ( other condition ) THEN ( desired output ) ELSE ( desired output ) END AS ( descriptive header for the output column ) FROM ( appropriate table ) In the example we see this done with ELSE City which puts LA unchanged in the new column Syntax NOTE: If you put the column name after THEN or ELSE it will put the value from the original column into the newly created column. AS: Used to set a specific name for the returned CASE column.END: Indicates the end of the CASE loop.If no ELSE statement is present and all WHEN conditions are false, the returned value will be NULL.ELSE: catches all of the entries that were not true for any of the WHEN conditions.If the condition is false, the next WHEN statement will be evaluated.After THEN is executed, CASE will return to the top of the loop and begin checking the next entry.THEN: executed when the condition is true, determines the output for the true condition.WHEN: indicates the start of a condition that should be checked by the query. ![]() CASE: indicates a condition loop has been started and that the conditions will follow.Let’s break down each component of the CASE WHEN statement: SELECT City, CASE WHEN City = "SF" THEN "San Francisco" ELSE City END AS "Updated City" FROM friends ![]()
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