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      SQL
        1  示例
            1.1  查看数据库元数据
                1.1.1  用户表名
SELECT TABLENAME FROM SYS.SYSTABLES WHERE TABLETYPE='T'
                1.1.2  用户表信息
SELECT * FROM SYS.SYSTABLES WHERE TABLETYPE='T'
                1.1.3  系统表名
SELECT TABLENAME FROM SYS.SYSTABLES WHERE TABLETYPE='S'
                1.1.4  系统表信息
SELECT * FROM SYS.SYSTABLES WHERE TABLETYPE='S'
                1.1.5  视图名
SELECT TABLENAME FROM SYS.SYSTABLES WHERE TABLETYPE='V'
                1.1.6  视图信息
SELECT * FROM SYS.SYSTABLES WHERE TABLETYPE='V'
                1.1.7  表的列名和类型
SELECT columnname, columndatatype FROM SYS.SYSTABLES t, SYS.SYSCOLUMNS c where t.TABLEID=c.REFERENCEID AND tablename='TREE' order by columnnumber
                1.1.8  表的列信息
SELECT c.* FROM SYS.SYSTABLES t, SYS.SYSCOLUMNS c where t.TABLEID=c.REFERENCEID AND tablename='TREE' order by columnnumber
                1.1.9  表的索引名
SELECT CONGLOMERATENAME FROM SYS.SYSTABLES t, SYS.SYSCONGLOMERATES c where t.TABLEID=c.TABLEID AND tablename='TREE'
                1.1.10  表的索引信息
SELECT c.* FROM SYS.SYSTABLES t, SYS.SYSCONGLOMERATES c where t.TABLEID=c.TABLEID AND tablename='TREE'
            1.2  查询数据库表
                1.2.1  行范围
SELECT * FROM visit_history OFFSET 2 ROWS FETCH NEXT 50 ROWS ONLY
                1.2.2  最多行数
SELECT * FROM visit_history FETCH FIRST 300 ROWS ONLY
                1.2.3  首行
SELECT * FROM visit_history FETCH FIRST ROW ONLY
                1.2.4  查询条件
SELECT * FROM visit_history WHERE file_type > 1 OR data_more like '%fxml%'
                1.2.5  计数
SELECT count(file_type) as count FROM visit_history WHERE file_type > 0 AND last_visit_time BETWEEN '2022-03-11 00:00:00.000' AND '2022-03-18 13:09:41.000'
                1.2.6  众数
SELECT file_type, count(file_type) AS mode FROM visit_history GROUP BY file_type ORDER BY mode DESC FETCH FIRST ROW ONLY

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