SQLite 3.27

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CREATE TABLE currencies (currency TEXT NOT NULL PRIMARY KEY);
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INSERT INTO currencies VALUES ('c1'), ('c2'), ('c3');
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CREATE TABLE table1 (currency TEXT NOT NULL PRIMARY KEY REFERENCES currencies(currency));
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INSERT INTO table1 VALUES ('c1'), ('c2'), ('c3');
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CREATE TABLE table2 (currency TEXT NOT NULL PRIMARY KEY REFERENCES currencies(currency));
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INSERT INTO table2 VALUES ('c1'), ('c2'), ('c3');
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CREATE TRIGGER remove_currencies_1 AFTER DELETE ON table1
BEGIN
  DELETE FROM currencies
  WHERE currency = OLD.currency
    AND NOT EXISTS (SELECT 1 FROM table2 WHERE currency = OLD.currency);
END;
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CREATE TRIGGER remove_currencies_2 AFTER DELETE ON table2
BEGIN
  DELETE FROM currencies
  WHERE currency = OLD.currency
    AND NOT EXISTS (SELECT 1 FROM table1 WHERE currency = OLD.currency);
END;
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DELETE FROM table1 WHERE currency = 'c2';
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SELECT * FROM currencies;


currency


c1

c2

c3


DELETE FROM table2 WHERE currency = 'c2';
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SELECT * FROM currencies;


currency


c1

c3


DELETE FROM table2 WHERE currency = 'c1';
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SELECT * FROM currencies;


currency


c1

c3


DELETE FROM table1 WHERE currency = 'c1';
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SELECT * FROM currencies;


currency


c3