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 times did Richard Durbin become a legislator in district 20? Richard Durbin is an official_full_name;  official_full_name refers to first_name, last_name; district 20 refers to district = 20

    【Evidence】
    Richard Durbin is an official_full_name;  official_full_name refers to first_name, last_name; district 20 refers to district = 20

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

SQL: SELECT SUM(CASE WHEN T2.district = 20 THEN 1 ELSE 0 END) AS count FROM current AS T1 INNER JOIN `current-terms` AS T2 ON T1.bioguide_id = T2.bioguide WHERE T1.first_name = 'Richard' AND T1.last_name = 'Durbin'