关于sql的case when用法简述

刚入手公司项目，需要添加一个功能，用到了SQL的case when以及concat

SELECT
        eve.cc,
        eve.sc,
        case concat(cc,sc)
        WHEN '' THEN ''
        WHEN '' THEN '得意先'
        WHEN '' THEN '仕入先'
        WHEN '' THEN '得意先/仕入先'
        ELSE ''
        END as client_supplier_class
        FROM ( SELECT bd0.id,
        bd0.created,
        bd0.creator,
        bd0.modified,
        bd0.modifier,
        bd0.layout_id,
        bd0.app_id,
        bd0.parent_id,
        bd0.revision,
        bd0.rev_ctrl_id,
        bd0.owner,
        bd0.owned_org,
        ct0.client_cd,
        bd0.name,
        ct0.client_charge_select,
        usr.so_usr_furigana,
        ct0.client_charge_cd,
        ct0.official_name,
        ct0.pronunciation,
        ct0.zip_cd,
        ct0.address_1,
        ct0.address_2,
    　　ct0.tel,
    　　ct0.tel2,
    　　ct0.fax,
    　　ct0.fax2,
    　　ct0.lower_limit_time,
    　　ct0.lower_limit_time_unit,
    　　ct0.upper_limit_time,
    　　ct0.upper_limit_time_unit,
    　　ct0.valid_start_date,
    　　ct0.valid_end_date,
    　　ct0.client_class as cc,
    　　ct0.supplier_class as sc
    FROM basedataset bd0
    LEFT JOIN sys_client_mst ct0 ON bd0.id::text = ct0.id::text
    LEFT JOIN ( SELECT v_user_all.id,
    v_user_all.furigana AS so_usr_furigana
    FROM v_user_all) usr ON usr.id::text = ct0.client_charge_select::text
    WHERE bd0.layout_id::text = 'sys_client_mst'::text) eve;