### Database Schema
CREATE TABLE "Country"
(
    CountryCode                                TEXT not null
        primary key,
    ShortName                                  TEXT,
    TableName                                  TEXT,
    LongName                                   TEXT,
    Alpha2Code                                 TEXT,
    CurrencyUnit                               TEXT,
    SpecialNotes                               TEXT,
    Region                                     TEXT,
    IncomeGroup                                TEXT,
    Wb2Code                                    TEXT,
    NationalAccountsBaseYear                   TEXT,
    NationalAccountsReferenceYear              TEXT,
    SnaPriceValuation                          TEXT,
    LendingCategory                            TEXT,
    OtherGroups                                TEXT,
    SystemOfNationalAccounts                   TEXT,
    AlternativeConversionFactor                TEXT,
    PppSurveyYear                              TEXT,
    BalanceOfPaymentsManualInUse               TEXT,
    ExternalDebtReportingStatus                TEXT,
    SystemOfTrade                              TEXT,
    GovernmentAccountingConcept                TEXT,
    ImfDataDisseminationStandard               TEXT,
    LatestPopulationCensus                     TEXT,
    LatestHouseholdSurvey                      TEXT,
    SourceOfMostRecentIncomeAndExpenditureData TEXT,
    VitalRegistrationComplete                  TEXT,
    LatestAgriculturalCensus                   TEXT,
    LatestIndustrialData                       INTEGER,
    LatestTradeData                            INTEGER,
    LatestWaterWithdrawalData                  INTEGER
)

CREATE TABLE "Series"
(
    SeriesCode                       TEXT not null
        primary key,
    Topic                            TEXT,
    IndicatorName                    TEXT,
    ShortDefinition                  TEXT,
    LongDefinition                   TEXT,
    UnitOfMeasure                    TEXT,
    Periodicity                      TEXT,
    BasePeriod                       TEXT,
    OtherNotes                       INTEGER,
    AggregationMethod                TEXT,
    LimitationsAndExceptions         TEXT,
    NotesFromOriginalSource          TEXT,
    GeneralComments                  TEXT,
    Source                           TEXT,
    StatisticalConceptAndMethodology TEXT,
    DevelopmentRelevance             TEXT,
    RelatedSourceLinks               TEXT,
    OtherWebLinks                    INTEGER,
    RelatedIndicators                INTEGER,
    LicenseType                      TEXT
)

CREATE TABLE CountryNotes
(
    Countrycode TEXT NOT NULL ,
    Seriescode  TEXT NOT NULL ,
    Description TEXT,
    primary key (Countrycode, Seriescode),
    FOREIGN KEY (Seriescode) REFERENCES Series(SeriesCode),
    FOREIGN KEY (Countrycode) REFERENCES Country(CountryCode)
)

CREATE TABLE Footnotes
(
    Countrycode TEXT NOT NULL ,
    Seriescode  TEXT NOT NULL ,
    Year        TEXT,
    Description TEXT,
    primary key (Countrycode, Seriescode, Year),
    FOREIGN KEY (Seriescode) REFERENCES Series(SeriesCode),
    FOREIGN KEY (Countrycode) REFERENCES Country(CountryCode)
)

CREATE TABLE Indicators
(
    CountryName   TEXT,
    CountryCode   TEXT NOT NULL ,
    IndicatorName TEXT,
    IndicatorCode TEXT NOT NULL ,
    Year          INTEGER NOT NULL ,
    Value         INTEGER,
    primary key (CountryCode, IndicatorCode, Year),
    FOREIGN KEY (CountryCode) REFERENCES Country(CountryCode)
)

CREATE TABLE SeriesNotes
(
    Seriescode  TEXT not null ,
    Year        TEXT not null ,
    Description TEXT,
    primary key (Seriescode, Year),
    foreign key (Seriescode) references Series(SeriesCode)
)

### Question
What is the average number of passengers carried via air transport per year by Bulgaria between 1970 to 1980? Indicate the country's system of trade.

### Hint
average number refers to avg(value); passengers carried via air transport per year refers to value where IndicatorName = 'Air transport, passengers carried'; by Bulgaria refers to CountryName = 'Bulgaria'; between 1970 to 1980 refers to Year between 1970 and 1980

### SQL
SELECT AVG(T1.Value), T2.SystemOfTrade FROM Indicators AS T1 INNER JOIN Country AS T2 ON T1.CountryCode = T2.CountryCode WHERE T1.IndicatorName = 'Air transport, passengers carried' AND T1.Year >= 1970 AND T1.Year < 1981 AND T1.CountryName = 'Bulgaria'