关于sql的case when用法简述

SELECT                                             
        eve.cc,                                                                    
        eve.sc,                                                                    
        case concat(cc,sc)                                                    
        WHEN ‘00‘ THEN ‘‘                                                            
        WHEN ‘10‘ THEN ‘得意先‘                                                    
        WHEN ‘01‘ THEN ‘仕入先‘
        WHEN ‘11‘ 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;