Is not equal to: <>
Empty condition is determined: null and blank (empty string)
Judging whether or not null: xxxx is not null / xxxx is null
Judgment null:
SELECT * FROM student WHERE address IS NULL ;
Analyzing empty string:
SELECT * FROM student WHERE address='';
And null empty string comprising:
SELECT * FROM student WHERE address IS NULL OR address='';
Fuzzy query: like
SELECT * FROM score WHERE sname LIKE 'Great%'; (% control multiple characters)
SELECT * FROM score WHERE sname LIKE 'big _'; (_ a control character)
SELECT * FROM score WHERE sname LIKE '__'; (two characters)
Aggregate queries: (maximum value) SUM (sum) AVG (average) max min (minimum) COUNT (total number of records)
SELECT SUM(js) FROM score WHERE sclass='j1018';
SELECT AVG(html) FROM score WHERE sclass='JAVA-1018';
SELECT MIN(html+js+jquery) FROM score;
SELECT COUNT (*) FROM score; (total statistics do less data: Each column count it, whichever is greater)
Sentenced empty function
IFNULL(html,0)；
SELECT MIN(IFNULL(html,0)+js+jquery) FROM score;