SQL

-- Upsert
INSERT INTO my_table(...) VALUES(...) ON CONFLICT DO UPDATE SET ...;

-- Find rows with duplicate values of column
SELECT t.* FROM my_table t
JOIN (
  SELECT my_col FROM my_table GROUP BY my_col HAVING COUNT(*) > 1
) dup ON t.my_col = dup.my_col
ORDER BY my_col

-- Pair adjacent rows using a window function
SELECT
  date AS end_date,
  LAG(date) OVER (ORDER BY date) AS start_date
FROM
  my_table

-- DDL
-- https://www.postgresql.org/docs/current/ddl-alter.html
ALTER TABLE my_table ADD COLUMN description TEXT NOT NULL;
ALTER TABLE my_table RENAME COLUMN my_column TO my_new_column;
ALTER TABLE my_table DROP COLUMN my_column;
ALTER TABLE my_table ADD CONSTRAINT my_constraint CHECK (...);
ALTER TABLE my_table ALTER COLUMN my_column DROP DEFAULT;