---
metadata:
  - name: generator
    content: Diplodoc Platform v5.52.0
alternate:
  - https://ydb.tech/docs/en/dev/yql-tutorial/conditional_values.md?version=v26.1
  - https://ydb.tech/docs/ru/dev/yql-tutorial/conditional_values.md?version=v26.1
  - href: en/dev/yql-tutorial/conditional_values.md
    type: text/markdown
    title: Markdown version
  - href: ../../llms.txt
    type: text/markdown
    title: llms.txt
sourcePath: en/core/dev/yql-tutorial/conditional_values.md
---
> **Documentation Index:** Fetch the complete configuration index at https://ydb.tech/docs/en/llms.txt

# Additional selection criteria

Select all the episode names of the first season of each series and sort them by name.

<!-- source: en/dev/yql-tutorial/_includes/yql_tutorial_prerequisites.md -->
{% note info %}

We assume that you already created tables in step [Creating a table](https://ydb.tech/docs/en/dev/yql-tutorial/create_demo_tables.md?version=v26.1) and populated them with data in step [Adding data to a table](https://ydb.tech/docs/en/dev/yql-tutorial/fill_tables_with_data.md?version=v26.1).

{% endnote %}
<!-- endsource: en/dev/yql-tutorial/_includes/yql_tutorial_prerequisites.md -->

```yql
SELECT
    series_title,               -- series_title is defined below in GROUP BY

    String::JoinFromList(       -- calling a C++ UDF,
                                -- see below

        AGGREGATE_LIST(title),  -- an aggregate function that
                                -- returns all the passed values as a list

        ", "                    -- String::JoinFromList concatenates
                                -- items of a given list (the first argument)
                                -- to a string using the separator (the second argument)
    ) AS episode_titles
FROM episodes
WHERE series_id IN (1,2)        -- IN defines the set of values in the WHERE clause,
                                -- to be included into the result.
                                -- Syntax:
                                -- test_expression (NOT) IN
                                -- ( subquery | expression ` ,...n ` )
                                -- If the value of test_expression is equal
                                -- to any value returned by subquery or is equal to
                                -- any expression from the comma-separated list,
                                -- the result value is TRUE. Otherwise, it's FALSE.
                                -- using NOT IN negates the result of subquery
                                -- or expression.
                                -- Warning: using null values together with
                                -- IN or NOT IN may lead to undesirable outcomes.
AND season_id = 1
GROUP BY
    CASE                        -- CASE evaluates a list of conditions and
                                -- returns one of multiple possible resulting
                                -- expressions. CASE can be used in any
                                -- statement or with any clause
                                -- that supports a given statement. For example, you can use CASE in
                                -- statements such as SELECT, UPDATE, and DELETE,
                                -- and in clauses such as IN, WHERE, and ORDER BY.
        WHEN series_id = 1
        THEN "IT Crowd"
        ELSE "Other series"
    END AS series_title         -- GROUP BY can be performed on
                                -- an arbitrary expression.
                                -- The result is available in a SELECT
                                -- via the alias specified with AS.
;

COMMIT;
```