Question

我试图在我的MySQL中执行一个简单的过程：

DELIMITER $$

CREATE PROCEDURE `countRows`(IN v varchar(30))
    BEGIN
    DECLARE e INT DEFAULT 0;
    SET @t1 =CONCAT("SELECT COUNT(*) FROM ",V, " INTO",e);
    PREPARE stmt3 FROM @t1;
    EXECUTE stmt3;
    DEALLOCATE PREPARE stmt3;
    SELECT CONCAT("The ",V," table contains ",e," rows");
   END$$
DELIMITER ;

编辑：一切正常但@ t1并没有将值保存在变量中，因此默认情况下第二个查询显示为0。

我无法看到任何语法错误，有什么建议让这个工作好吗？

Answer 1

@t2中的陈述;不需要预准备语句，因为它不包含任何要执行的SQL语句。下面提到的3个代码行是不必要的。

PREPARE stmt3 FROM @t2;
EXECUTE stmt3;
DEALLOCATE PREPARE stmt3;

您只需将其显示为

即可

SET @t2 =CONCAT("The ",V," table contains ",e," rows");
SELECT @t2;

（或）只是

SELECT CONCAT("The ",V," table contains ",e," rows");

这样，你的程序看起来应该是

DELIMITER $$
CREATE PROCEDURE `countRows`(IN v varchar(30))
BEGIN
DECLARE e INT DEFAULT 0;
SET @t1 =CONCAT("SELECT COUNT(*) FROM ",V, " INTO",e);
SET @t2 =CONCAT("The ",V," table contains ",e," rows");
PREPARE stmt3 FROM @t1;
EXECUTE stmt3;
DEALLOCATE PREPARE stmt3;
SELECT @t2;
END$$
DELIMITER ;

修改

尝试使用用户定义的变量（下面的过程体）

DELIMITER $$ CREATE PROCEDURE `countRows`(IN v varchar(30)) BEGIN SET @e := 0; SET @t1 =CONCAT("SELECT COUNT(*) INTO @e FROM ",V); PREPARE stmt3 FROM @t1; EXECUTE stmt3; DEALLOCATE PREPARE stmt3; SELECT CONCAT("The ",V," table contains ",@e," rows"); END$$ DELIMITER ;

Answer 2

正如您所指出的那样，您在存储过程中声明了变量# Add population boxplot (not grouped by peer) ggplot(dat, aes(year, pct, group=interaction(year,peer), colour=factor(peer))) + geom_boxplot(aes(group=year), width=0.05, colour="grey60", fill="#FFFFFF90") + geom_boxplot(position=position_dodge(width=0.4), width=0.2) + stat_summary(fun.y=mean, geom="line", position=position_dodge(width=0.4), aes(group=peer)) + stat_summary(fun.y=mean, geom="point", position=position_dodge(width=0.4), size=4, aes(group=peer)) + scale_x_continuous(breaks=unique(dat$year))，因此它不存在于该环境之外。

我认为问题出在这里：

# Add an ID variable to the data dat$id = rep(1:(nrow(dat)/2), each=2) library(gridExtra) # For grid.arrange function pdf("plots.pdf", 7, 5) for (i in unique(dat$id)) { p1 = ggplot() + geom_boxplot(data=dat[dat$peer==unique(dat$peer[dat$id==i]),], aes(year, pct, group=year)) + geom_point(data=dat[dat$id==i,], aes(year, pct), pch=8, colour="red", size=5) + ggtitle("Your Peers") p2 = ggplot() + geom_boxplot(data=dat, aes(year, pct, group=year)) + geom_point(data=dat[dat$id==i,], aes(year, pct), pch=8, colour="red", size=5) + ggtitle("All Participants") grid.arrange(p1, p2, ncol=2, main=paste0("ID = ", i)) } dev.off()

尝试将其更改为：

e

如何在过程MYSQL中声明变量

2 个答案: