MYSQL-- subquery

Subquery of MYSQL

1 find the average value

SELECT AVG (field such as good_price) FROM tdb_ table name

2 round the result of the average value and keep the two places after the decimal point

SELECT ROUND (AVG (field),) FROM tdb_ table name

3 which of the average prices in the query field are greater than or equal to the average I want to compare

The first kind of SELECT field, field, field FROM tdb_ table name WHERE price field > = average

The second subquery SELECT field, field, field FROM tdb_ table name WHERE price field > = (SELECT ROUND (AVG (field),) FROM tdb_ table name)

In fact, multi-table update is divided into three steps. Categories in the query table 2. Write the query results to data Table 3. Multi-table update

4 categories in the query table

SELECT column name (such as goods_cate) FROM tdb_ table name GROUP BY column name

5 write the query results to the data table

INSERT tdb_ table name _ cates (cate_name) SELECT column name FROM tdb_ table name GROUP BY column name

6 multiple table updates (update your own table with reference to another table)

Step 1: UPDATE tdb_ table name INNER JOIN tdb_ column name (e.g. goods_cates) ON column name (goods_cate) = column name (cate_name); (click to confirm)

Step 2 SET column name (good_cate) = column name (cate_id)