1、同字段多条件查询使用 case ，避免重复使用查询，例如
SELECT COUNT(*) FROM products WHERE price < 13;
COUNT(*)
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2
SELECT COUNT(*) FROM products WHERE price BETWEEN 13 AND 15;
COUNT(*)
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5
SELECT COUNT(*) FROM products WHERE price > 15;
COUNT(*)
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5
应换成case查询：如下
SELECT
COUNT(CASE WHEN price < 13 THEN 1 ELSE null END) low,
COUNT(CASE WHEN price BETWEEN 13 AND 15 THEN 1 ELSE null END) med,
COUNT(CASE WHEN price > 15 THEN 1 ELSE null END) high
FROM products;
LOW MED HIGH
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