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

    【Schema】
    【DB_ID】 legislator
【Schema】
# Table: current-terms
[
(address:TEXT),
(bioguide:TEXT, Primary Key, Examples: [A000055, A000360, A000367]),
(caucus:TEXT, Examples: [Democrat]),
(chamber:TEXT, Examples: [senate, house]),
(class:REAL, Examples: [1.0, 2.0, 3.0]),
(contact_form:TEXT),
(district:REAL, Examples: [13.0, 1.0, 3.0]),
(end:TEXT, Primary Key, Examples: [1999-01-03, 2001-01-03, 2003-01-03]),
(fax:TEXT, Examples: [202-228-6321, 202-228-0514, 202-224-1651]),
(last:TEXT, Examples: [Menendez]),
(name:TEXT, Examples: [Stewart Lee Udall, Morris K. Udall, Mark Udall]),
(office:TEXT, Examples: [713 Hart Senate Office Building]),
(party:TEXT, Examples: [Democrat, Republican, Independent]),
(party_affiliations:TEXT),
(phone:TEXT, Examples: [202-224-2315, 202-224-3441, 202-224-4524]),
(relation:TEXT, Examples: [son, nephew, cousin]),
(rss_url:TEXT),
(start:TEXT, Examples: [1993-01-05, 1995-01-04, 1997-01-07]),
(state:TEXT, Examples: [OH, WA, MD]),
(state_rank:TEXT, Examples: [senior, junior]),
(title:TEXT, Examples: [Minority Whip, Majority Whip, Minority Leader]),
(type:TEXT, Examples: [rep, sen]),
(url:TEXT)
]
# Table: historical
[
(ballotpedia_id:TEXT, Examples: [Mo Cowan, Jo Bonner, Rodney Alexander]),
(bioguide_id:TEXT, Primary Key, Examples: [A000001, A000002, A000003]),
(bioguide_previous_id:TEXT, Examples: [['F000246'], ['L000266'], ['W000790']]),
(birthday_bio:TEXT, Examples: [1745-04-02, 1742-03-21, 1743-06-16]),
(cspan_id:TEXT, Examples: [12590.0, 4479.0, 103.0]),
(fec_id:TEXT, Examples: [['S6CO00168'], ['S2MN00126'], ['S0ID00057']]),
(first_name:TEXT, Examples: [Richard, Theodorick, Aedanus]),
(gender_bio:TEXT, Examples: [M, F]),
(google_entity_id_id:TEXT, Examples: [kg:/m/02pz46, kg:/m/033mf4, kg:/m/03yccv]),
(govtrack_id:INTEGER, Examples: [401222, 401521, 402032]),
(house_history_alternate_id:TEXT, Examples: [13283.0]),
(house_history_id:REAL, Examples: [9479.0, 10177.0, 10687.0]),
(icpsr_id:REAL, Examples: [507.0, 786.0, 1260.0]),
(last_name:TEXT, Examples: [Bassett, Bland, Burke]),
(lis_id:TEXT, Examples: [S134, S135, S138]),
(maplight_id:TEXT, Examples: [232.0, 581.0, 585.0]),
(middle_name:TEXT, Examples: [Samuel, Henry, Jenifer]),
(nickname_name:TEXT, Examples: [of Carrollton, of Colchester, of Milton]),
(official_full_name:TEXT, Examples: [Enid Greene Waldholtz]),
(opensecrets_id:TEXT, Examples: [N00008333, N00006953, N00009160]),
(religion_bio:TEXT, Examples: [Baptist, Roman Catholic, Methodist]),
(suffix_name:TEXT, Examples: [Jr., Sr., II]),
(thomas_id:TEXT, Examples: [01308, 01270, 01281]),
(votesmart_id:TEXT, Examples: [52156.0, 128775.0, 26829.0]),
(wikidata_id:TEXT, Examples: [Q518823, Q1749152, Q380504]),
(wikipedia_id:TEXT, Examples: [Richard Bassett (politician)])
]
# Table: current
[
(ballotpedia_id:TEXT, Examples: [Sherrod Brown, Maria Cantwell, Ben Cardin]),
(bioguide_id:TEXT, Primary Key, Examples: [A000055, A000360, A000367]),
(birthday_bio:DATE, Examples: [1952-11-09]),
(cspan_id:REAL, Primary Key, Examples: [45516.0, 5.0, 1033767.0]),
(fec_id:TEXT, Examples: [['H2OH13033', 'S6OH00163']]),
(first_name:TEXT, Examples: [Sherrod, Maria, Benjamin]),
(gender_bio:TEXT, Examples: [M, F]),
(google_entity_id_id:TEXT, Examples: [kg:/m/034s80, kg:/m/01x68t, kg:/m/025k3k]),
(govtrack_id:INTEGER, Examples: [400050, 300018, 400064]),
(house_history_id:REAL, Examples: [9996.0, 10608.0, 10629.0]),
(icpsr_id:REAL, Examples: [29389.0, 39310.0, 15408.0]),
(last_name:TEXT, Examples: [Brown, Cantwell, Cardin]),
(lis_id:TEXT, Examples: [S307, S275, S308]),
(maplight_id:REAL, Examples: [168.0, 544.0, 182.0]),
(middle_name:TEXT, Examples: [L., Richard, P.]),
(nickname_name:TEXT, Examples: [Bob, Bernie, Jim]),
(official_full_name:TEXT, Examples: [Sherrod Brown, Maria Cantwell, Benjamin L. Cardin]),
(opensecrets_id:TEXT, Examples: [N00003535, N00007836, N00001955]),
(religion_bio:TEXT, Examples: [Lutheran, Roman Catholic, Jewish]),
(suffix_name:TEXT, Examples: [Jr., III, II]),
(thomas_id:INTEGER, Examples: [136, 172, 174]),
(votesmart_id:REAL, Examples: [27018.0, 27122.0, 26888.0]),
(wikidata_id:TEXT, Examples: [Q381880, Q22250, Q723295]),
(wikipedia_id:TEXT, Examples: [Sherrod Brown, Maria Cantwell, Ben Cardin])
]
# Table: social-media
[
(bioguide:TEXT, Primary Key, Examples: [A000055, A000360, A000367]),
(facebook:TEXT, Examples: [congresswomanaumuaamata]),
(facebook_id:REAL, Examples: [1537155909907320.0, 186203844738421.0, 351671691660938.0]),
(govtrack:REAL, Examples: [412664.0, 412428.0, 412667.0]),
(instagram:TEXT, Examples: [senjoniernst, senatorwarner, SenWhitehouse]),
(instagram_id:REAL, Examples: [1582702853.0, 515327940.0, 1559860900.0]),
(thomas:INTEGER, Examples: [2222, 2019, 2283]),
(twitter:TEXT, Examples: [RepAmata, RepToddYoung, SenJoniErnst]),
(twitter_id:REAL, Examples: [3026622545.0, 234128524.0, 2856787757.0]),
(youtube:TEXT, Examples: [RepToddYoung, RepYoder, RepJohnYarmuth]),
(youtube_id:TEXT, Examples: [UCGdrLQbt1PYDTPsampx4t1A])
]
# Table: historical-terms
[
(address:TEXT),
(bioguide:TEXT, Primary Key, Examples: [A000001, A000002, A000003]),
(chamber:TEXT, Examples: [house, senate]),
(class:REAL, Examples: [2.0, 1.0, 3.0]),
(contact_form:TEXT),
(district:REAL, Examples: [9.0, 2.0, 6.0]),
(end:TEXT, Examples: [1793-03-03, 1791-03-03, 1795-03-03]),
(fax:TEXT, Examples: [202-228-6363, 202-226-6299, 202-226-0893]),
(last:TEXT, Examples: [Levy, Lambert, Bono]),
(middle:TEXT),
(name:TEXT),
(office:TEXT, Examples: [248 Russell Senate Office Building]),
(party:TEXT, Examples: [Anti-Administration, Pro-Administration, Republican]),
(party_affiliations:TEXT),
(phone:TEXT, Examples: [202-224-4024, 202-224-4543, 202-225-2706]),
(relation:TEXT),
(rss_url:TEXT),
(start:TEXT, Examples: [1789-03-04, 1790-01-01, 1791-10-24]),
(state:TEXT, Examples: [DE, VA, SC]),
(state_rank:TEXT, Examples: [junior, senior]),
(title:TEXT, Examples: [Majority Leader, Speaker]),
(type:TEXT, Examples: [sen, rep]),
(url:TEXT)
]
【Foreign keys】
current-terms.bioguide=current.bioguide_id
social-media.bioguide=current.bioguide_id
historical-terms.bioguide=historical.bioguide_id

    【Question】
    How many districts did John Conyers, Jr. serve in total?

    【Evidence】
    

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

SQL: SELECT COUNT(T3.district) FROM ( SELECT T2.district FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.official_full_name = 'John Conyers, Jr.' GROUP BY T2.district ) T3