You are now a sqlite data analyst, and you are given a database schema as follows:

    【Schema】
    【DB_ID】 mondial_geo
【Schema】
# Table: lake
[
(Name:TEXT, Primary Key, Examples: [Ammersee, Arresoe, Atlin Lake]),
(Area:REAL, Examples: [46.6, 40.2, 798.0]),
(Depth:REAL, Examples: [81.1, 5.6, 283.0]),
(Altitude:REAL, Examples: [533.0, 668.0, 104.0]),
(Type:TEXT, Examples: [artificial, salt, caldera]),
(River:TEXT, Examples: [Ammer, Yukon River, Sanaga]),
(Longitude:REAL, Examples: [11.6, 12.1, -133.75]),
(Latitude:REAL, Examples: [48.0, 56.0, 59.5])
]
# Table: geo_island
[
(Island:TEXT, Primary Key, Examples: [Aland, Alicudi, Ambon]),
(Country:TEXT, Primary Key, Examples: [Alan, Lipa, Molu]),
(Province:TEXT, Primary Key, Examples: [101, 102, 10244])
]
# Table: sea
[
(Name:TEXT, Primary Key, Examples: [Andaman Sea, Arabian Sea, Arctic Ocean]),
(Depth:REAL, Examples: [3113.0, 5203.0, 5608.0])
]
# Table: island
[
(Name:TEXT, Primary Key, Examples: [Aland, Alicudi, Ambon]),
(Islands:TEXT, Examples: [Aland Islands, Lipari Islands, Moluccan Islands]),
(Area:REAL, Examples: [650.0, 5.2, 775.0]),
(Height:REAL, Examples: [675.0, 1225.0, 32.0]),
(Type:TEXT, Examples: [volcanic, lime, coral]),
(Longitude:REAL, Examples: [20.0, 14.4, 128.2]),
(Latitude:REAL, Examples: [60.1, 38.6, -3.7])
]
# Table: city
[
(Name:TEXT, Primary Key, Examples: [Aachen, Aalborg, Aarau]),
(Country:TEXT, Examples: [D, DK, CH]),
(Province:TEXT, Primary Key, Examples: [Nordrhein Westfalen, Denmark, AG]),
(Population:INTEGER, Examples: [247113, 113865, 194345]),
(Longitude:REAL, Examples: [10.0, 10.1, -4.0]),
(Latitude:REAL, Examples: [57.0, 56.1, 5.3])
]
# Table: ethnicGroup
[
(Country:TEXT, Primary Key, Examples: [GE, EAU, DJI]),
(Name:TEXT, Primary Key, Examples: [Abkhaz, Acholi, Afar]),
(Percentage:REAL, Examples: [1.8, 4.0, 35.0])
]
# Table: country
[
(Name:TEXT, Examples: [Afghanistan, Albania, Algeria]),
(Code:TEXT, Primary Key, Examples: [A, AFG, AG]),
(Capital:TEXT, Examples: [Vienna, Kabul, Saint Johns]),
(Province:TEXT, Examples: [Vienna, Afghanistan, Antigua and Barbuda]),
(Area:REAL, Examples: [83850.0, 647500.0, 442.0]),
(Population:INTEGER, Examples: [8023244, 22664136, 65647])
]
# Table: locatedOn
[
(City:TEXT, Primary Key, Examples: [Aberdeen, Aberystwyth, Adamstown]),
(Province:TEXT, Primary Key, Examples: [Province, Grampian, Ceredigion]),
(Country:TEXT, Primary Key, Examples: [Coun, GB, PITC]),
(Island:TEXT, Primary Key, Examples: [Island, Great Britain, Pitcairn])
]
# Table: river
[
(Name:TEXT, Primary Key, Examples: [Aare, Adda, Akagera]),
(River:TEXT, Examples: [Rhein, Po, Ohio River]),
(Lake:TEXT, Examples: [Brienzersee, Lago di Como, Lake Victoria]),
(Sea:TEXT, Examples: [Atlantic Ocean, Sea of Okhotsk, Mediterranean Sea]),
(Length:REAL, Examples: [288.0, 313.0, 275.0]),
(SourceLongitude:REAL, Examples: [8.2, 10.3, 29.3]),
(SourceLatitude:REAL, Examples: [46.55, -2.5, 41.9]),
(Mountains:TEXT, Examples: [Alps]),
(SourceAltitude:REAL, Examples: [2310.0, 2235.0, 2700.0]),
(EstuaryLongitude:REAL, Examples: [8.22, 9.88, 33.0]),
(EstuaryLatitude:REAL, Examples: [47.61, 45.13, -1.0])
]
# Table: geo_mountain
[
(Mountain:TEXT, Primary Key, Examples: [Mountain, Gran Sasso, Tirich Mir]),
(Country:TEXT, Primary Key, Examples: [Coun, I, AFG]),
(Province:TEXT, Primary Key, Examples: [Abruzzo, Afghanistan, Agri])
]
# Table: located
[
(City:TEXT, Examples: [City, Shkoder, Durres]),
(Province:TEXT, Examples: [Province]),
(Country:TEXT, Examples: [Coun, AL, GR]),
(River:TEXT, Examples: [River, Donau, Save]),
(Lake:TEXT, Examples: [Lake, Lake Skutari, Bodensee]),
(Sea:TEXT, Examples: [Sea, Mediterranean Sea, Atlantic Ocean])
]
# Table: mountainOnIsland
[
(Mountain:TEXT, Primary Key, Examples: [Andringitra, Asahi-Dake, Barbeau Peak]),
(Island:TEXT, Primary Key, Examples: [Island, Madagaskar, Hokkaido])
]
# Table: isMember
[
(Country:TEXT, Primary Key, Examples: [A, AFG, AG]),
(Organization:TEXT, Primary Key, Examples: [AG, ANC, AfDB]),
(Type:TEXT, Examples: [Type, nonregional member, observer])
]
# Table: borders
[
(Country1:TEXT, Primary Key, Examples: [A, AFG, AL]),
(Country2:TEXT, Primary Key, Examples: [CH, CZ, D]),
(Length:REAL, Examples: [164.0, 362.0, 784.0])
]
# Table: religion
[
(Country:TEXT, Primary Key, Examples: [BERM, AUS, AXA]),
(Name:TEXT, Primary Key, Examples: [African Methodist Episcopal]),
(Percentage:REAL, Examples: [11.0, 26.1, 29.0])
]
# Table: mountain
[
(Name:TEXT, Primary Key),
(Mountains:TEXT),
(Height:REAL),
(Type:TEXT),
(Longitude:REAL),
(Latitude:REAL)
]
# Table: population
[
(Country:TEXT, Primary Key, Examples: [A, AFG, AG]),
(Population_Growth:REAL, Examples: [0.41, 4.78, 0.76]),
(Infant_Mortality:REAL, Examples: [6.2, 149.7, 17.2])
]
# Table: geo_source
[
(River:TEXT, Primary Key, Examples: [River, White Nile, Amudarja]),
(Country:TEXT, Primary Key, Examples: [Coun, SUD, AFG]),
(Province:TEXT, Primary Key, Examples: [Aali an Nil, Afghanistan, Agri])
]
# Table: organization
[
(Abbreviation:TEXT, Primary Key, Examples: [ABEDA, ACC, ACCT]),
(Name:TEXT, Examples: [ASEAN-Mekong Basin Development Group]),
(City:TEXT, Examples: [City, Khartoum, Paris]),
(Country:TEXT, Examples: [Coun, SUD, F]),
(Province:TEXT, Examples: [Province, al Khartum, Ile de France]),
(Established:DATE)
]
# Table: desert
[
(Name:TEXT, Primary Key, Examples: [Arabian Desert, Atacama, Azaouad]),
(Area:REAL, Examples: [50000.0, 181300.0, 80000.0]),
(Longitude:REAL, Examples: [26.0, -69.25, 0.0]),
(Latitude:REAL, Examples: [33.0, -24.5, 20.0])
]
# Table: politics
[
(Country:TEXT, Primary Key, Examples: [A, AFG, AG]),
(Independence:DATE),
(Dependent:TEXT, Examples: [Depe, USA, NL]),
(Government:TEXT, Examples: [Government])
]
# Table: mergesWith
[
(Sea1:TEXT, Primary Key, Examples: [Andaman Sea, Arabian Sea, Arctic Ocean]),
(Sea2:TEXT, Primary Key, Examples: [Sea2, Gulf of Bengal, Indian Ocean])
]
# Table: encompasses
[
(Country:TEXT, Primary Key, Examples: [A, AFG, AG]),
(Continent:TEXT, Primary Key, Examples: [Europe, Asia, America]),
(Percentage:REAL, Examples: [100.0, 90.0, 10.0])
]
# Table: continent
[
(Name:TEXT, Primary Key, Examples: [Africa, America, Asia]),
(Area:REAL, Examples: [30254700.0, 39872000.0, 45095300.0])
]
# Table: province
[
(Name:TEXT, Primary Key, Examples: [AG, AI, AR]),
(Country:TEXT, Primary Key, Examples: [CH, SUD, GB]),
(Population:INTEGER, Examples: [1599605, 110700, 1263000]),
(Area:REAL, Examples: [238792.0, 1130.0, 10794.0]),
(Capital:TEXT, Examples: [Malakal, Colwyn Bay, LAquila]),
(CapProv:TEXT, Examples: [Aali an Nil, Aberconwy and Colwyn, Abruzzo])
]
# Table: geo_river
[
(River:TEXT, Primary Key, Examples: [River]),
(Country:TEXT, Primary Key, Examples: [Coun, SUD, IRQ]),
(Province:TEXT, Primary Key, Examples: [AG, Aali an Nil, Ad Tamim])
]
# Table: language
[
(Country:TEXT, Primary Key, Examples: [AFG, NAM, MK]),
(Name:TEXT, Primary Key, Examples: [Afghan Persian, Afrikaans, Albanian]),
(Percentage:REAL, Examples: [50.0, 60.0, 21.0])
]
# Table: economy
[
(Country:TEXT, Primary Key, Examples: [A, AFG, AG]),
(GDP:REAL, Examples: [152000.0, 12800.0, 425.0]),
(Agriculture:REAL, Examples: [2.0, 65.0, 3.5]),
(Service:REAL, Examples: [34.0, 15.0, 19.3]),
(Industry:REAL, Examples: [64.0, 20.0, 77.2]),
(Inflation:REAL, Examples: [2.3, 3.5, 16.0])
]
# Table: islandIn
[
(Island:TEXT, Examples: [Island, Svalbard, Greenland]),
(Sea:TEXT, Examples: [Sea, Norwegian Sea, Barents Sea]),
(Lake:TEXT, Examples: [Lake, Ozero Baikal, Lake Toba]),
(River:TEXT, Examples: [River])
]
# Table: target
[
(Country:TEXT, Primary Key, Examples: [A, AFG, AL]),
(Target:TEXT, Examples: [Target, Christian, non-Christian])
]
# Table: geo_desert
[
(Desert:TEXT, Primary Key, Examples: [Desert, Rub Al Chali, Dascht-e-Margoh]),
(Country:TEXT, Primary Key, Examples: [Coun, UAE, AFG]),
(Province:TEXT, Primary Key, Examples: [Abu Dhabi, Afghanistan, Ahal])
]
# Table: geo_estuary
[
(River:TEXT, Primary Key, Examples: [River]),
(Country:TEXT, Primary Key, Examples: [Coun, SUD, AFG]),
(Province:TEXT, Primary Key, Examples: [AG, Aali an Nil, Afghanistan])
]
# Table: geo_sea
[
(Sea:TEXT, Primary Key, Examples: [Sea, Mediterranean Sea, Persian Gulf]),
(Country:TEXT, Primary Key, Examples: [Coun, I, UAE]),
(Province:TEXT, Primary Key, Examples: [Abruzzo, Abu Dhabi, Adana])
]
# Table: geo_lake
[
(Lake:TEXT, Primary Key, Examples: [Lake, Barrage de Mbakaou, Lake Nicaragua]),
(Country:TEXT, Primary Key, Examples: [Coun, CAM, CR]),
(Province:TEXT, Primary Key, Examples: [Adamaoua, Alajuela, Albania])
]
【Foreign keys】
geo_island.Province=province.Name
geo_island.Country=province.Country
geo_island.Country=country.Code
geo_island.Island=island.Name
city.Province=province.Name
city.Country=province.Country
city.Country=country.Code
ethnicGroup.Country=country.Code
locatedOn.Province=province.Name
locatedOn.Country=province.Country
locatedOn.City=city.Name
locatedOn.Province=city.Province
locatedOn.Island=island.Name
locatedOn.Country=country.Code
river.Lake=lake.Name
geo_mountain.Province=province.Name
geo_mountain.Country=province.Country
geo_mountain.Country=country.Code
geo_mountain.Mountain=mountain.Name
located.Province=province.Name
located.Country=province.Country
located.City=city.Name
located.Province=city.Province
located.Sea=sea.Name
located.Lake=lake.Name
located.River=river.Name
located.Country=country.Code
mountainOnIsland.Island=island.Name
mountainOnIsland.Mountain=mountain.Name
isMember.Organization=organization.Abbreviation
isMember.Country=country.Code
borders.Country2=country.Code
borders.Country1=country.Code
religion.Country=country.Code
population.Country=country.Code
geo_source.Province=province.Name
geo_source.Country=province.Country
geo_source.Country=country.Code
geo_source.River=river.Name
organization.Province=province.Name
organization.Country=province.Country
organization.City=city.Name
organization.Province=city.Province
organization.Country=country.Code
politics.Dependent=country.Code
politics.Country=country.Code
mergesWith.Sea2=sea.Name
mergesWith.Sea1=sea.Name
encompasses.Continent=continent.Name
encompasses.Country=country.Code
province.Country=country.Code
geo_river.Province=province.Name
geo_river.Country=province.Country
geo_river.Country=country.Code
geo_river.River=river.Name
language.Country=country.Code
economy.Country=country.Code
islandIn.River=river.Name
islandIn.Lake=lake.Name
islandIn.Sea=sea.Name
islandIn.Island=island.Name
target.Country=country.Code
geo_desert.Province=province.Name
geo_desert.Country=province.Country
geo_desert.Country=country.Code
geo_desert.Desert=desert.Name
geo_estuary.Province=province.Name
geo_estuary.Country=province.Country
geo_estuary.Country=country.Code
geo_estuary.River=river.Name
geo_sea.Province=province.Name
geo_sea.Country=province.Country
geo_sea.Country=country.Code
geo_sea.Sea=sea.Name
geo_lake.Province=province.Name
geo_lake.Country=province.Country
geo_lake.Country=country.Code
geo_lake.Lake=lake.Name

    【Question】
    Average length of the rivers flowing into the Donau River.

    【Evidence】
    

    Please read and understand the database schema carefully, and generate an executable SQL based on the user's question and evidence. 

SQL: SELECT * FROM river WHERE Name = 'Donau'