##要在一个操作中更改两个现有列的类型
ALTER TABLE "hbird"."company_main" ALTER COLUMN legal_person TYPE varchar(64);
##要将一个varchar类型的列添加到表中
ALTER TABLE "hbird"."company_main" ADD COLUMN legal_person varchar(30);
##从表中删除列
ALTER TABLE "hbird"."company_main" DROP COLUMN legal_person RESTRICT;
##重命名现有列
ALTER TABLE "hbird"."company_main" RENAME COLUMN address TO city;
##重命名现有表
ALTER TABLE "hbird"."company_main" RENAME TO company_mains;
查询sql执行效率结构
EXPLAIN SELECT * FROM company_main
查询重复的企业名称
SELECT * FROM "hbird"."company_main" a WHERE ((SELECT COUNT(*) FROM "hbird"."company_main" WHERE name = a.name) > 1) ORDER BY name DESC
查询重复的法人名
SELECT * FROM "hbird"."company_main" a WHERE ((SELECT COUNT(*) FROM "hbird"."company_main" WHERE legal_person = a.legal_person) > 1) ORDER BY legal_person DESC
统计重复的企业有多少个
SELECT name,count(name) as count FROM "hbird"."company_main" GROUP BY name HAVING count(name) >1 ORDER BY count DESC;