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SELECT 'ID: ', id, col_1 + col_2, sqrt(col_2)
FROM   t1
-- precedence within WHERE: functions, comparisons, NOT, AND, OR
WHERE  col_1 > 100
AND    NOT MOD(col_2, 10) = 0
OR     col_3 < col_1
ORDER  BY col_4 DESC, col_5;

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-- number of rows, number of not-null-values
SELECT COUNT(*), COUNT(col_1) FROM t1;

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-- predefined functions
SELECT COUNT(col_1), MAX(col_1), MIN(col_1), AVG(col_1), SUM(col_1) FROM t1;

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-- UNIQUE values only
SELECT DISTINCT col_1 FROM t1;

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-- Only the combination of col_1 plus col_2 is unique
SELECT DISTINCT col_1, col_2 FROM t1;

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SELECT id,
       CASE contact_type
         WHEN 'fixed line' THEN 'Phone'
         WHEN 'mobile'     THEN 'Phone'
         ELSE                   'Not a telephone number'
       END AS 'contact_type',
       contact_value
FROM   contact;